diff --git a/src/Entity/Attachments/AttachmentContainingDBElement.php b/src/Entity/Attachments/AttachmentContainingDBElement.php
index b550394e..e74d7cb6 100644
--- a/src/Entity/Attachments/AttachmentContainingDBElement.php
+++ b/src/Entity/Attachments/AttachmentContainingDBElement.php
@@ -46,7 +46,7 @@ abstract class AttachmentContainingDBElement extends AbstractNamedDBElement impl
      * Mapping is done in sub classes like part
      * @Groups({"full"})
      */
-    protected $attachments;
+    protected Collection $attachments;
 
     public function __construct()
     {
diff --git a/src/Entity/Attachments/AttachmentType.php b/src/Entity/Attachments/AttachmentType.php
index 33ed8b18..de67e8d9 100644
--- a/src/Entity/Attachments/AttachmentType.php
+++ b/src/Entity/Attachments/AttachmentType.php
@@ -45,7 +45,7 @@ class AttachmentType extends AbstractStructuralDBElement
      * @ORM\OneToMany(targetEntity="AttachmentType", mappedBy="parent", cascade={"persist"})
      * @ORM\OrderBy({"name" = "ASC"})
      */
-    protected $children;
+    protected Collection $children;
 
     /**
      * @ORM\ManyToOne(targetEntity="AttachmentType", inversedBy="children")
@@ -65,14 +65,14 @@ class AttachmentType extends AbstractStructuralDBElement
      * @ORM\OrderBy({"name" = "ASC"})
      * @Assert\Valid()
      */
-    protected $attachments;
+    protected Collection $attachments;
 
     /** @var Collection<int, AttachmentTypeParameter>
      * @ORM\OneToMany(targetEntity="App\Entity\Parameters\AttachmentTypeParameter", mappedBy="element", cascade={"persist", "remove"}, orphanRemoval=true)
      * @ORM\OrderBy({"group" = "ASC" ,"name" = "ASC"})
      * @Assert\Valid()
      */
-    protected $parameters;
+    protected Collection $parameters;
 
     /**
      * @var Collection<int, Attachment>
diff --git a/src/Entity/Base/AbstractPartsContainingDBElement.php b/src/Entity/Base/AbstractPartsContainingDBElement.php
index be0b230a..52326907 100644
--- a/src/Entity/Base/AbstractPartsContainingDBElement.php
+++ b/src/Entity/Base/AbstractPartsContainingDBElement.php
@@ -22,6 +22,7 @@ declare(strict_types=1);
 
 namespace App\Entity\Base;
 
+use Doctrine\Common\Collections\Collection;
 use Doctrine\ORM\Mapping as ORM;
 use Symfony\Component\Serializer\Annotation\Groups;
 
@@ -33,5 +34,5 @@ use Symfony\Component\Serializer\Annotation\Groups;
 abstract class AbstractPartsContainingDBElement extends AbstractStructuralDBElement
 {
     /** @Groups({"full"}) */
-    protected $parameters;
+    protected Collection $parameters;
 }
diff --git a/src/Entity/Base/AbstractStructuralDBElement.php b/src/Entity/Base/AbstractStructuralDBElement.php
index eec1a73e..2ac874ca 100644
--- a/src/Entity/Base/AbstractStructuralDBElement.php
+++ b/src/Entity/Base/AbstractStructuralDBElement.php
@@ -87,7 +87,7 @@ abstract class AbstractStructuralDBElement extends AttachmentContainingDBElement
      * @var AbstractStructuralDBElement[]|Collection
      * @Groups({"include_children"})
      */
-    protected $children;
+    protected Collection $children;
 
     /**
      * @var AbstractStructuralDBElement
@@ -106,6 +106,7 @@ abstract class AbstractStructuralDBElement extends AttachmentContainingDBElement
         parent::__construct();
         $this->children = new ArrayCollection();
         $this->parameters = new ArrayCollection();
+        $this->parent = null;
     }
 
     public function __clone()
diff --git a/src/Entity/LabelSystem/LabelProfile.php b/src/Entity/LabelSystem/LabelProfile.php
index 46c478ee..12f6d659 100644
--- a/src/Entity/LabelSystem/LabelProfile.php
+++ b/src/Entity/LabelSystem/LabelProfile.php
@@ -61,7 +61,7 @@ class LabelProfile extends AttachmentContainingDBElement
      * @ORM\OneToMany(targetEntity="App\Entity\Attachments\LabelAttachment", mappedBy="element", cascade={"persist", "remove"}, orphanRemoval=true)
      * @ORM\OrderBy({"name" = "ASC"})
      */
-    protected $attachments;
+    protected Collection $attachments;
 
     /**
      * @var LabelOptions
diff --git a/src/Entity/Parameters/AbstractParameter.php b/src/Entity/Parameters/AbstractParameter.php
index 08e4d7b7..922bb43e 100644
--- a/src/Entity/Parameters/AbstractParameter.php
+++ b/src/Entity/Parameters/AbstractParameter.php
@@ -143,7 +143,7 @@ abstract class AbstractParameter extends AbstractNamedDBElement
      *
      * @var AbstractDBElement|null the element to which this parameter belongs to
      */
-    protected ?AbstractDBElement $element;
+    protected ?AbstractDBElement $element = null;
 
     public function __construct()
     {
diff --git a/src/Entity/Parameters/AttachmentTypeParameter.php b/src/Entity/Parameters/AttachmentTypeParameter.php
index 89fdbbc3..8a161883 100644
--- a/src/Entity/Parameters/AttachmentTypeParameter.php
+++ b/src/Entity/Parameters/AttachmentTypeParameter.php
@@ -42,6 +42,7 @@ declare(strict_types=1);
 namespace App\Entity\Parameters;
 
 use App\Entity\Attachments\AttachmentType;
+use App\Entity\Base\AbstractDBElement;
 use Doctrine\ORM\Mapping as ORM;
 use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
 
@@ -57,5 +58,5 @@ class AttachmentTypeParameter extends AbstractParameter
      * @ORM\ManyToOne(targetEntity="App\Entity\Attachments\AttachmentType", inversedBy="parameters")
      * @ORM\JoinColumn(name="element_id", referencedColumnName="id", nullable=false, onDelete="CASCADE").
      */
-    protected ?\App\Entity\Base\AbstractDBElement $element;
+    protected ?AbstractDBElement $element = null;
 }
diff --git a/src/Entity/Parameters/CategoryParameter.php b/src/Entity/Parameters/CategoryParameter.php
index f5989823..cce90a86 100644
--- a/src/Entity/Parameters/CategoryParameter.php
+++ b/src/Entity/Parameters/CategoryParameter.php
@@ -41,6 +41,7 @@ declare(strict_types=1);
 
 namespace App\Entity\Parameters;
 
+use App\Entity\Base\AbstractDBElement;
 use App\Entity\Parts\Category;
 use Doctrine\ORM\Mapping as ORM;
 use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
@@ -57,5 +58,5 @@ class CategoryParameter extends AbstractParameter
      * @ORM\ManyToOne(targetEntity="App\Entity\Parts\Category", inversedBy="parameters")
      * @ORM\JoinColumn(name="element_id", referencedColumnName="id", nullable=false, onDelete="CASCADE").
      */
-    protected ?\App\Entity\Base\AbstractDBElement $element;
+    protected ?AbstractDBElement $element = null;
 }
diff --git a/src/Entity/Parameters/CurrencyParameter.php b/src/Entity/Parameters/CurrencyParameter.php
index 4afdedbe..9f99310e 100644
--- a/src/Entity/Parameters/CurrencyParameter.php
+++ b/src/Entity/Parameters/CurrencyParameter.php
@@ -41,6 +41,7 @@ declare(strict_types=1);
 
 namespace App\Entity\Parameters;
 
+use App\Entity\Base\AbstractDBElement;
 use App\Entity\PriceInformations\Currency;
 use Doctrine\ORM\Mapping as ORM;
 use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
@@ -60,5 +61,5 @@ class CurrencyParameter extends AbstractParameter
      * @ORM\ManyToOne(targetEntity="App\Entity\PriceInformations\Currency", inversedBy="parameters")
      * @ORM\JoinColumn(name="element_id", referencedColumnName="id", nullable=false, onDelete="CASCADE").
      */
-    protected ?\App\Entity\Base\AbstractDBElement $element;
+    protected ?AbstractDBElement $element = null;
 }
diff --git a/src/Entity/Parameters/FootprintParameter.php b/src/Entity/Parameters/FootprintParameter.php
index aa6f803c..cb1dd185 100644
--- a/src/Entity/Parameters/FootprintParameter.php
+++ b/src/Entity/Parameters/FootprintParameter.php
@@ -41,6 +41,7 @@ declare(strict_types=1);
 
 namespace App\Entity\Parameters;
 
+use App\Entity\Base\AbstractDBElement;
 use App\Entity\Parts\Footprint;
 use Doctrine\ORM\Mapping as ORM;
 use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
@@ -58,5 +59,5 @@ class FootprintParameter extends AbstractParameter
      * @ORM\ManyToOne(targetEntity="App\Entity\Parts\Footprint", inversedBy="parameters")
      * @ORM\JoinColumn(name="element_id", referencedColumnName="id", nullable=false, onDelete="CASCADE").
      */
-    protected ?\App\Entity\Base\AbstractDBElement $element;
+    protected ?AbstractDBElement $element = null;
 }
diff --git a/src/Entity/Parameters/GroupParameter.php b/src/Entity/Parameters/GroupParameter.php
index 0d22ae62..81a0d39c 100644
--- a/src/Entity/Parameters/GroupParameter.php
+++ b/src/Entity/Parameters/GroupParameter.php
@@ -41,6 +41,7 @@ declare(strict_types=1);
 
 namespace App\Entity\Parameters;
 
+use App\Entity\Base\AbstractDBElement;
 use App\Entity\UserSystem\Group;
 use Doctrine\ORM\Mapping as ORM;
 use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
@@ -58,5 +59,5 @@ class GroupParameter extends AbstractParameter
      * @ORM\ManyToOne(targetEntity="App\Entity\UserSystem\Group", inversedBy="parameters")
      * @ORM\JoinColumn(name="element_id", referencedColumnName="id", nullable=false, onDelete="CASCADE").
      */
-    protected ?\App\Entity\Base\AbstractDBElement $element;
+    protected ?AbstractDBElement $element = null;
 }
diff --git a/src/Entity/Parameters/ManufacturerParameter.php b/src/Entity/Parameters/ManufacturerParameter.php
index 547e3b80..56d785f2 100644
--- a/src/Entity/Parameters/ManufacturerParameter.php
+++ b/src/Entity/Parameters/ManufacturerParameter.php
@@ -41,6 +41,7 @@ declare(strict_types=1);
 
 namespace App\Entity\Parameters;
 
+use App\Entity\Base\AbstractDBElement;
 use App\Entity\Parts\Manufacturer;
 use Doctrine\ORM\Mapping as ORM;
 use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
@@ -58,5 +59,5 @@ class ManufacturerParameter extends AbstractParameter
      * @ORM\ManyToOne(targetEntity="App\Entity\Parts\Manufacturer", inversedBy="parameters")
      * @ORM\JoinColumn(name="element_id", referencedColumnName="id", nullable=false, onDelete="CASCADE").
      */
-    protected ?\App\Entity\Base\AbstractDBElement $element;
+    protected ?AbstractDBElement $element = null;
 }
diff --git a/src/Entity/Parameters/MeasurementUnitParameter.php b/src/Entity/Parameters/MeasurementUnitParameter.php
index c95f8844..e4980431 100644
--- a/src/Entity/Parameters/MeasurementUnitParameter.php
+++ b/src/Entity/Parameters/MeasurementUnitParameter.php
@@ -41,6 +41,7 @@ declare(strict_types=1);
 
 namespace App\Entity\Parameters;
 
+use App\Entity\Base\AbstractDBElement;
 use App\Entity\Parts\MeasurementUnit;
 use Doctrine\ORM\Mapping as ORM;
 use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
@@ -58,5 +59,5 @@ class MeasurementUnitParameter extends AbstractParameter
      * @ORM\ManyToOne(targetEntity="App\Entity\Parts\MeasurementUnit", inversedBy="parameters")
      * @ORM\JoinColumn(name="element_id", referencedColumnName="id", nullable=false, onDelete="CASCADE").
      */
-    protected ?\App\Entity\Base\AbstractDBElement $element;
+    protected ?AbstractDBElement $element = null;
 }
diff --git a/src/Entity/Parameters/PartParameter.php b/src/Entity/Parameters/PartParameter.php
index b5d0fbaa..a3845e55 100644
--- a/src/Entity/Parameters/PartParameter.php
+++ b/src/Entity/Parameters/PartParameter.php
@@ -41,6 +41,7 @@ declare(strict_types=1);
 
 namespace App\Entity\Parameters;
 
+use App\Entity\Base\AbstractDBElement;
 use App\Entity\Parts\Part;
 use Doctrine\ORM\Mapping as ORM;
 use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
@@ -58,5 +59,5 @@ class PartParameter extends AbstractParameter
      * @ORM\ManyToOne(targetEntity="App\Entity\Parts\Part", inversedBy="parameters")
      * @ORM\JoinColumn(name="element_id", referencedColumnName="id", nullable=false, onDelete="CASCADE").
      */
-    protected ?\App\Entity\Base\AbstractDBElement $element;
+    protected ?AbstractDBElement $element = null;
 }
diff --git a/src/Entity/Parameters/ProjectParameter.php b/src/Entity/Parameters/ProjectParameter.php
index b4269e9c..d4f8cd1a 100644
--- a/src/Entity/Parameters/ProjectParameter.php
+++ b/src/Entity/Parameters/ProjectParameter.php
@@ -41,6 +41,7 @@ declare(strict_types=1);
 
 namespace App\Entity\Parameters;
 
+use App\Entity\Base\AbstractDBElement;
 use App\Entity\ProjectSystem\Project;
 use Doctrine\ORM\Mapping as ORM;
 use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
@@ -58,5 +59,5 @@ class ProjectParameter extends AbstractParameter
      * @ORM\ManyToOne(targetEntity="App\Entity\ProjectSystem\Project", inversedBy="parameters")
      * @ORM\JoinColumn(name="element_id", referencedColumnName="id", nullable=false, onDelete="CASCADE").
      */
-    protected ?\App\Entity\Base\AbstractDBElement $element;
+    protected ?AbstractDBElement $element = null;
 }
diff --git a/src/Entity/Parameters/StorelocationParameter.php b/src/Entity/Parameters/StorelocationParameter.php
index b236ad1c..6c7d430b 100644
--- a/src/Entity/Parameters/StorelocationParameter.php
+++ b/src/Entity/Parameters/StorelocationParameter.php
@@ -41,6 +41,7 @@ declare(strict_types=1);
 
 namespace App\Entity\Parameters;
 
+use App\Entity\Base\AbstractDBElement;
 use App\Entity\Parts\Storelocation;
 use Doctrine\ORM\Mapping as ORM;
 use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
@@ -58,5 +59,5 @@ class StorelocationParameter extends AbstractParameter
      * @ORM\ManyToOne(targetEntity="App\Entity\Parts\Storelocation", inversedBy="parameters")
      * @ORM\JoinColumn(name="element_id", referencedColumnName="id", nullable=false, onDelete="CASCADE").
      */
-    protected ?\App\Entity\Base\AbstractDBElement $element;
+    protected ?AbstractDBElement $element = null;
 }
diff --git a/src/Entity/Parameters/SupplierParameter.php b/src/Entity/Parameters/SupplierParameter.php
index 534cc949..6acac705 100644
--- a/src/Entity/Parameters/SupplierParameter.php
+++ b/src/Entity/Parameters/SupplierParameter.php
@@ -41,6 +41,7 @@ declare(strict_types=1);
 
 namespace App\Entity\Parameters;
 
+use App\Entity\Base\AbstractDBElement;
 use App\Entity\Parts\Supplier;
 use Doctrine\ORM\Mapping as ORM;
 use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
@@ -58,5 +59,5 @@ class SupplierParameter extends AbstractParameter
      * @ORM\ManyToOne(targetEntity="App\Entity\Parts\Supplier", inversedBy="parameters")
      * @ORM\JoinColumn(name="element_id", referencedColumnName="id", nullable=false, onDelete="CASCADE").
      */
-    protected ?\App\Entity\Base\AbstractDBElement $element;
+    protected ?AbstractDBElement $element = null;
 }
diff --git a/src/Entity/Parts/Category.php b/src/Entity/Parts/Category.php
index 7dd37c0f..8a79d871 100644
--- a/src/Entity/Parts/Category.php
+++ b/src/Entity/Parts/Category.php
@@ -24,6 +24,7 @@ namespace App\Entity\Parts;
 
 use App\Entity\Attachments\CategoryAttachment;
 use App\Entity\Base\AbstractPartsContainingDBElement;
+use App\Entity\Base\AbstractStructuralDBElement;
 use App\Entity\Parameters\CategoryParameter;
 use Doctrine\Common\Collections\Collection;
 use Doctrine\ORM\Mapping as ORM;
@@ -46,13 +47,13 @@ class Category extends AbstractPartsContainingDBElement
      * @ORM\OrderBy({"name" = "ASC"})
      * @var Collection
      */
-    protected $children;
+    protected Collection $children;
 
     /**
      * @ORM\ManyToOne(targetEntity="Category", inversedBy="children")
      * @ORM\JoinColumn(name="parent_id", referencedColumnName="id")
      */
-    protected ?\App\Entity\Base\AbstractStructuralDBElement $parent;
+    protected ?AbstractStructuralDBElement $parent = null;
 
     /**
      * @var string
@@ -109,6 +110,7 @@ class Category extends AbstractPartsContainingDBElement
      * @Groups({"full", "import"})
      */
     protected string $default_comment = '';
+
     /**
      * @var Collection<int, CategoryAttachment>
      * @ORM\OneToMany(targetEntity="App\Entity\Attachments\CategoryAttachment", mappedBy="element", cascade={"persist", "remove"}, orphanRemoval=true)
@@ -116,7 +118,7 @@ class Category extends AbstractPartsContainingDBElement
      * @Assert\Valid()
      * @Groups({"full"})
      */
-    protected $attachments;
+    protected Collection $attachments;
 
     /** @var Collection<int, CategoryParameter>
      * @ORM\OneToMany(targetEntity="App\Entity\Parameters\CategoryParameter", mappedBy="element", cascade={"persist", "remove"}, orphanRemoval=true)
@@ -124,7 +126,7 @@ class Category extends AbstractPartsContainingDBElement
      * @Assert\Valid()
      * @Groups({"full"})
      */
-    protected $parameters;
+    protected Collection $parameters;
 
     public function getPartnameHint(): string
     {
diff --git a/src/Entity/Parts/Footprint.php b/src/Entity/Parts/Footprint.php
index 079d1168..09a4f4d0 100644
--- a/src/Entity/Parts/Footprint.php
+++ b/src/Entity/Parts/Footprint.php
@@ -51,7 +51,7 @@ class Footprint extends AbstractPartsContainingDBElement
      * @ORM\OrderBy({"name" = "ASC"})
      * @var Collection
      */
-    protected $children;
+    protected Collection $children;
 
     /**
      * @var Collection<int, FootprintAttachment>
@@ -59,7 +59,7 @@ class Footprint extends AbstractPartsContainingDBElement
      * @ORM\OrderBy({"name" = "ASC"})
      * @Assert\Valid()
      */
-    protected $attachments;
+    protected Collection $attachments;
 
     /**
      * @var FootprintAttachment|null
@@ -73,7 +73,7 @@ class Footprint extends AbstractPartsContainingDBElement
      * @ORM\OrderBy({"group" = "ASC" ,"name" = "ASC"})
      * @Assert\Valid()
      */
-    protected $parameters;
+    protected Collection $parameters;
 
     /****************************************
      * Getters
diff --git a/src/Entity/Parts/Manufacturer.php b/src/Entity/Parts/Manufacturer.php
index 17ab8543..74d0e308 100644
--- a/src/Entity/Parts/Manufacturer.php
+++ b/src/Entity/Parts/Manufacturer.php
@@ -51,7 +51,7 @@ class Manufacturer extends AbstractCompany
      * @ORM\OrderBy({"name" = "ASC"})
      * @var Collection
      */
-    protected $children;
+    protected Collection $children;
 
     /**
      * @var Collection<int, ManufacturerAttachment>
@@ -59,12 +59,12 @@ class Manufacturer extends AbstractCompany
      * @ORM\OrderBy({"name" = "ASC"})
      * @Assert\Valid()
      */
-    protected $attachments;
+    protected Collection $attachments;
 
     /** @var Collection<int, ManufacturerParameter>
      * @ORM\OneToMany(targetEntity="App\Entity\Parameters\ManufacturerParameter", mappedBy="element", cascade={"persist", "remove"}, orphanRemoval=true)
      * @ORM\OrderBy({"group" = "ASC" ,"name" = "ASC"})
      * @Assert\Valid()
      */
-    protected $parameters;
+    protected Collection $parameters;
 }
diff --git a/src/Entity/Parts/MeasurementUnit.php b/src/Entity/Parts/MeasurementUnit.php
index 7ebdfc85..b33d47fc 100644
--- a/src/Entity/Parts/MeasurementUnit.php
+++ b/src/Entity/Parts/MeasurementUnit.php
@@ -75,7 +75,7 @@ class MeasurementUnit extends AbstractPartsContainingDBElement
      * @ORM\OrderBy({"name" = "ASC"})
      * @var Collection
      */
-    protected $children;
+    protected Collection $children;
 
     /**
      * @ORM\ManyToOne(targetEntity="MeasurementUnit", inversedBy="children")
@@ -89,14 +89,14 @@ class MeasurementUnit extends AbstractPartsContainingDBElement
      * @ORM\OrderBy({"name" = "ASC"})
      * @Assert\Valid()
      */
-    protected $attachments;
+    protected Collection $attachments;
 
     /** @var Collection<int, MeasurementUnitParameter>
      * @ORM\OneToMany(targetEntity="App\Entity\Parameters\MeasurementUnitParameter", mappedBy="element", cascade={"persist", "remove"}, orphanRemoval=true)
      * @ORM\OrderBy({"group" = "ASC" ,"name" = "ASC"})
      * @Assert\Valid()
      */
-    protected $parameters;
+    protected Collection $parameters;
 
     /**
      * @return string
diff --git a/src/Entity/Parts/Part.php b/src/Entity/Parts/Part.php
index 4cbdb7a0..d1179782 100644
--- a/src/Entity/Parts/Part.php
+++ b/src/Entity/Parts/Part.php
@@ -75,7 +75,7 @@ class Part extends AttachmentContainingDBElement
      * @ORM\OrderBy({"group" = "ASC" ,"name" = "ASC"})
      * @Groups({"full"})
      */
-    protected $parameters;
+    protected Collection $parameters;
 
     /**
      * @ORM\Column(type="datetime", name="datetime_added", options={"default":"CURRENT_TIMESTAMP"})
@@ -100,7 +100,7 @@ class Part extends AttachmentContainingDBElement
      * @Assert\Valid()
      * @Groups({"full"})
      */
-    protected $attachments;
+    protected Collection $attachments;
 
     /**
      * @var DateTime|null the date when this element was modified the last time
diff --git a/src/Entity/Parts/Storelocation.php b/src/Entity/Parts/Storelocation.php
index 58e2f2e8..03c5951f 100644
--- a/src/Entity/Parts/Storelocation.php
+++ b/src/Entity/Parts/Storelocation.php
@@ -24,6 +24,7 @@ namespace App\Entity\Parts;
 
 use App\Entity\Attachments\StorelocationAttachment;
 use App\Entity\Base\AbstractPartsContainingDBElement;
+use App\Entity\Base\AbstractStructuralDBElement;
 use App\Entity\Parameters\StorelocationParameter;
 use App\Entity\UserSystem\User;
 use Doctrine\Common\Collections\Collection;
@@ -47,13 +48,13 @@ class Storelocation extends AbstractPartsContainingDBElement
      * @ORM\OrderBy({"name" = "ASC"})
      * @var Collection
      */
-    protected $children;
+    protected Collection $children;
 
     /**
      * @ORM\ManyToOne(targetEntity="Storelocation", inversedBy="children")
      * @ORM\JoinColumn(name="parent_id", referencedColumnName="id")
      */
-    protected ?\App\Entity\Base\AbstractStructuralDBElement $parent;
+    protected ?AbstractStructuralDBElement $parent;
 
     /**
      * @var MeasurementUnit|null The measurement unit, which parts can be stored in here
@@ -67,7 +68,7 @@ class Storelocation extends AbstractPartsContainingDBElement
      * @ORM\OrderBy({"group" = "ASC" ,"name" = "ASC"})
      * @Assert\Valid()
      */
-    protected $parameters;
+    protected Collection $parameters;
 
     /**
      * @var bool
@@ -109,7 +110,7 @@ class Storelocation extends AbstractPartsContainingDBElement
      * @ORM\OneToMany(targetEntity="App\Entity\Attachments\StorelocationAttachment", mappedBy="element", cascade={"persist", "remove"}, orphanRemoval=true)
      * @Assert\Valid()
      */
-    protected $attachments;
+    protected Collection $attachments;
 
     /********************************************************************************
      *
diff --git a/src/Entity/Parts/Supplier.php b/src/Entity/Parts/Supplier.php
index e6068115..f8ab4202 100644
--- a/src/Entity/Parts/Supplier.php
+++ b/src/Entity/Parts/Supplier.php
@@ -24,6 +24,7 @@ namespace App\Entity\Parts;
 
 use App\Entity\Attachments\SupplierAttachment;
 use App\Entity\Base\AbstractCompany;
+use App\Entity\Base\AbstractStructuralDBElement;
 use App\Entity\Parameters\SupplierParameter;
 use App\Entity\PriceInformations\Currency;
 use App\Validator\Constraints\BigDecimal\BigDecimalPositiveOrZero;
@@ -50,18 +51,18 @@ class Supplier extends AbstractCompany
      * @ORM\OrderBy({"name" = "ASC"})
      * @var Collection
      */
-    protected $children;
+    protected Collection $children;
 
     /**
      * @ORM\ManyToOne(targetEntity="Supplier", inversedBy="children")
      * @ORM\JoinColumn(name="parent_id", referencedColumnName="id")
      */
-    protected ?\App\Entity\Base\AbstractStructuralDBElement $parent;
+    protected ?AbstractStructuralDBElement $parent;
 
     /**
      * @ORM\OneToMany(targetEntity="App\Entity\PriceInformations\Orderdetail", mappedBy="supplier")
      */
-    protected $orderdetails;
+    protected Collection $orderdetails;
 
     /**
      * @var Currency|null The currency that should be used by default for order informations with this supplier.
@@ -86,14 +87,14 @@ class Supplier extends AbstractCompany
      * @ORM\OrderBy({"name" = "ASC"})
      * @Assert\Valid()
      */
-    protected $attachments;
+    protected Collection $attachments;
 
     /** @var Collection<int, SupplierParameter>
      * @ORM\OneToMany(targetEntity="App\Entity\Parameters\SupplierParameter", mappedBy="element", cascade={"persist", "remove"}, orphanRemoval=true)
      * @ORM\OrderBy({"group" = "ASC" ,"name" = "ASC"})
      * @Assert\Valid()
      */
-    protected $parameters;
+    protected Collection $parameters;
 
     /**
      * Gets the currency that should be used by default, when creating a orderdetail with this supplier.
diff --git a/src/Entity/PriceInformations/Currency.php b/src/Entity/PriceInformations/Currency.php
index 4f6e0923..6b78de19 100644
--- a/src/Entity/PriceInformations/Currency.php
+++ b/src/Entity/PriceInformations/Currency.php
@@ -69,7 +69,7 @@ class Currency extends AbstractStructuralDBElement
      * @ORM\OneToMany(targetEntity="Currency", mappedBy="parent", cascade={"persist"})
      * @ORM\OrderBy({"name" = "ASC"})
      */
-    protected $children;
+    protected Collection $children;
 
     /**
      * @ORM\ManyToOne(targetEntity="Currency", inversedBy="children")
@@ -83,19 +83,19 @@ class Currency extends AbstractStructuralDBElement
      * @ORM\OrderBy({"name" = "ASC"})
      * @Assert\Valid()
      */
-    protected $attachments;
+    protected Collection $attachments;
 
     /** @var Collection<int, CurrencyParameter>
      * @ORM\OneToMany(targetEntity="App\Entity\Parameters\CurrencyParameter", mappedBy="element", cascade={"persist", "remove"}, orphanRemoval=true)
      * @ORM\OrderBy({"group" = "ASC" ,"name" = "ASC"})
      * @Assert\Valid()
      */
-    protected $parameters;
+    protected Collection $parameters;
 
     /** @var Collection<int, Pricedetail>
      * @ORM\OneToMany(targetEntity="App\Entity\PriceInformations\Pricedetail", mappedBy="currency")
      */
-    protected $pricedetails;
+    protected Collection $pricedetails;
 
     public function __construct()
     {
diff --git a/src/Entity/ProjectSystem/Project.php b/src/Entity/ProjectSystem/Project.php
index ab0f2739..0ffcbfe8 100644
--- a/src/Entity/ProjectSystem/Project.php
+++ b/src/Entity/ProjectSystem/Project.php
@@ -47,7 +47,7 @@ class Project extends AbstractStructuralDBElement
      * @ORM\OrderBy({"name" = "ASC"})
      * @var Collection
      */
-    protected $children;
+    protected Collection $children;
 
     /**
      * @ORM\ManyToOne(targetEntity="Project", inversedBy="children")
@@ -98,13 +98,13 @@ class Project extends AbstractStructuralDBElement
      * @ORM\OneToMany(targetEntity="App\Entity\Attachments\ProjectAttachment", mappedBy="element", cascade={"persist", "remove"}, orphanRemoval=true)
      * @ORM\OrderBy({"name" = "ASC"})
      */
-    protected $attachments;
+    protected Collection $attachments;
 
     /** @var Collection<int, ProjectParameter>
      * @ORM\OneToMany(targetEntity="App\Entity\Parameters\ProjectParameter", mappedBy="element", cascade={"persist", "remove"}, orphanRemoval=true)
      * @ORM\OrderBy({"group" = "ASC" ,"name" = "ASC"})
      */
-    protected $parameters;
+    protected Collection $parameters;
 
     /********************************************************************************
      *
diff --git a/src/Entity/UserSystem/Group.php b/src/Entity/UserSystem/Group.php
index e07d6fad..08976c30 100644
--- a/src/Entity/UserSystem/Group.php
+++ b/src/Entity/UserSystem/Group.php
@@ -49,7 +49,7 @@ class Group extends AbstractStructuralDBElement implements HasPermissionsInterfa
      * @ORM\OrderBy({"name" = "ASC"})
      * @var Collection
      */
-    protected $children;
+    protected Collection $children;
 
     /**
      * @ORM\ManyToOne(targetEntity="Group", inversedBy="children")
@@ -61,7 +61,7 @@ class Group extends AbstractStructuralDBElement implements HasPermissionsInterfa
      * @ORM\OneToMany(targetEntity="User", mappedBy="group")
      * @var Collection<User>
      */
-    protected $users;
+    protected Collection $users;
 
     /**
      * @var bool If true all users associated with this group must have enabled some kind of 2 factor authentication
@@ -75,7 +75,7 @@ class Group extends AbstractStructuralDBElement implements HasPermissionsInterfa
      * @ORM\OrderBy({"name" = "ASC"})
      * @Assert\Valid()
      */
-    protected $attachments;
+    protected Collection $attachments;
 
     /**
      * @var PermissionData|null
@@ -90,7 +90,7 @@ class Group extends AbstractStructuralDBElement implements HasPermissionsInterfa
      * @ORM\OrderBy({"group" = "ASC" ,"name" = "ASC"})
      * @Assert\Valid()
      */
-    protected $parameters;
+    protected Collection $parameters;
 
     public function __construct()
     {
diff --git a/src/Entity/UserSystem/User.php b/src/Entity/UserSystem/User.php
index 77182159..b809d717 100644
--- a/src/Entity/UserSystem/User.php
+++ b/src/Entity/UserSystem/User.php
@@ -226,7 +226,7 @@ class User extends AttachmentContainingDBElement implements UserInterface, HasPe
      * @ORM\OneToMany(targetEntity="App\Entity\Attachments\UserAttachment", mappedBy="element", cascade={"persist", "remove"}, orphanRemoval=true)
      * @ORM\OrderBy({"name" = "ASC"})
      */
-    protected $attachments;
+    protected Collection $attachments;
 
     /** @var DateTime|null The time when the backup codes were generated
      * @ORM\Column(type="datetime", nullable=true)
@@ -237,13 +237,13 @@ class User extends AttachmentContainingDBElement implements UserInterface, HasPe
     /** @var Collection<int, LegacyU2FKeyInterface>
      * @ORM\OneToMany(targetEntity="App\Entity\UserSystem\U2FKey", mappedBy="user", cascade={"REMOVE"}, orphanRemoval=true)
      */
-    protected $u2fKeys;
+    protected Collection $u2fKeys;
 
     /**
      * @var Collection<int, WebauthnKey>
      * @ORM\OneToMany(targetEntity="App\Entity\UserSystem\WebauthnKey", mappedBy="user", cascade={"REMOVE"}, orphanRemoval=true)
      */
-    protected $webauthn_keys;
+    protected Collection $webauthn_keys;
 
     /**
      * @var Currency|null The currency the user wants to see prices in.
diff --git a/src/Helpers/Trees/TreeViewNode.php b/src/Helpers/Trees/TreeViewNode.php
index f43a1eb6..85053239 100644
--- a/src/Helpers/Trees/TreeViewNode.php
+++ b/src/Helpers/Trees/TreeViewNode.php
@@ -36,11 +36,11 @@ final class TreeViewNode implements JsonSerializable
 
     private ?TreeViewNodeState $state = null;
 
-    private ?array $tags;
+    private ?array $tags = null;
 
-    private ?int $id;
+    private ?int $id = null;
 
-    private ?string $icon;
+    private ?string $icon = null;
 
     /**
      * Creates a new TreeView node with the given parameters.
diff --git a/src/Repository/UserRepository.php b/src/Repository/UserRepository.php
index 6913dea4..3cc29788 100644
--- a/src/Repository/UserRepository.php
+++ b/src/Repository/UserRepository.php
@@ -35,7 +35,7 @@ use Symfony\Component\Security\Core\User\UserInterface;
  */
 final class UserRepository extends NamedDBElementRepository implements PasswordUpgraderInterface
 {
-    protected ?User $anonymous_user;
+    protected ?User $anonymous_user = null;
 
     /**
      * Returns the anonymous user.