refactor: IHierarchyObject Nullable IGameManager to Non-Nullable

Since in a typical use case of these classes they are already in the
hierarchy and it bloats the behaviour code with lots of null checks

Engine.Core/Behaviour.cs

@@ -6,6 +6,7 @@ public abstract class Behaviour : BehaviourBase
 {
     private bool isInitializedThisFrame = false;
 
+    protected IGameManager GameManager => BehaviourController.GameObject.GameManager;
     protected IGameObject GameObject => BehaviourController.GameObject;
     protected ITransform Transform => BehaviourController.GameObject.Transform;
 
@@ -33,7 +34,7 @@ public abstract class Behaviour : BehaviourBase
         OnInitialize();
 
         if (GameObject.IsInHierarchy)
-            EnteredHierarchy(GameObject, GameObject.GameManager ?? throw new System.Exception("Unexpected Error"));
+            EnteredHierarchy(GameObject, GameManager);
     }
 
     protected virtual void OnFinalize() { }
@@ -48,7 +49,7 @@ public abstract class Behaviour : BehaviourBase
         OnFinalize();
 
         if (GameObject.IsInHierarchy)
-            ExitedHierarchy(GameObject, GameObject.GameManager ?? throw new System.Exception("Unexpected Error"));
+            ExitedHierarchy(GameObject, GameManager);
     }
 
     protected virtual void OnPreUpdatePreActiveCheck() { }