Use the BoxColliderNode class for handling actor spawn events.

Source/Editor/Modules/SceneModule.cs

@@ -534,8 +534,6 @@ namespace FlaxEditor.Modules
             {
                 node.ParentNode = parentNode;
             }
-
-            actor.OnActorSpawned();
         }
 
         private void OnActorDeleted(Actor actor)

Source/Editor/SceneGraph/Actors/BoxColliderNode.cs

@@ -8,8 +8,33 @@ using Real = System.Single;
 
 using FlaxEngine;
 
+#if FLAX_EDITOR
+using FlaxEditor.CustomEditors.Dedicated;
+using FlaxEditor.CustomEditors;
+#endif
+
 namespace FlaxEditor.SceneGraph.Actors
 {
+#if FLAX_EDITOR
+    /// <summary>
+    /// Dedicated custom editor for BoxCollider objects.
+    /// </summary>
+    [CustomEditor(typeof(BoxCollider)), DefaultEditor]
+    public class BoxColliderEditor : ActorEditor
+    {
+        /// <inheritdoc />
+        public override void Initialize(LayoutElementsContainer layout)
+        {
+            base.Initialize(layout);
+            layout.Space(20f);
+            var autoResizeButton = layout.Button("Resize to Fit", "Resize the box collider to fit it's parent's bounds.");
+
+            BoxCollider collider = Values[0] as BoxCollider;
+            autoResizeButton.Button.Clicked += collider.AutoResize;
+        }
+    }
+#endif
+
     /// <summary>
     /// Scene tree node for <see cref="BoxCollider"/> actor type.
     /// </summary>
@@ -37,5 +62,13 @@ namespace FlaxEditor.SceneGraph.Actors
 
             return base.RayCastSelf(ref ray, out distance, out normal);
         }
+
+        /// <inheritdoc />
+        public override void PostSpawn()
+        {
+            base.PostSpawn();
+            BoxCollider boxCollider = Actor as BoxCollider;
+            boxCollider.AutoResize();
+        }
     }
 }

Source/Engine/Level/Actor.cs

@@ -116,15 +116,6 @@ namespace FlaxEngine
             LocalTransform = Transform.Identity;
         }
 
-        /// <summary>
-        /// Called in-editor when an actor is added to the scene.
-        /// If not in the editor, this function will not be called.
-        /// </summary>
-        public virtual void OnActorSpawned()
-        {
-
-        }
-
         /// <summary>
         /// Creates a new child actor of the given type.
         /// </summary>

Source/Engine/Level/Actors/BoxCollider.cs

@@ -5,26 +5,6 @@ using FlaxEditor.CustomEditors.Dedicated;
 
 namespace FlaxEngine
 {
-#if FLAX_EDITOR
-    /// <summary>
-    /// Dedicated custom editor for BoxCollider objects.
-    /// </summary>
-    [CustomEditor(typeof(BoxCollider)), DefaultEditor]
-    public class BoxColliderEditor : ActorEditor
-    {
-        /// <inheritdoc />
-        public override void Initialize(LayoutElementsContainer layout)
-        {
-            base.Initialize(layout);
-            layout.Space(20f);
-            var autoResizeButton = layout.Button("Resize to Fit", "Resize the box collider to fit it's parent's bounds.");
-
-            BoxCollider collider = Values[0] as BoxCollider;
-            autoResizeButton.Button.Clicked += collider.AutoResize;
-        }
-    }
-#endif
-
     partial class BoxCollider
     {
         private void BoxExcluding(Actor target, ref BoundingBox output, Actor excluded)
@@ -72,12 +52,5 @@ namespace FlaxEngine
             // Undo Rotation
             Orientation *= Quaternion.Invert(Orientation);
         }
-
-        /// <inheritdoc />
-        public override void OnActorSpawned()
-        {
-            base.OnActorSpawned();
-            AutoResize();
-        }
     }
 }