fix: NetcodeIntegrationTest destroy GameObjects not NetworkObjects during shutdown sequence (#1756)

* fix

Destroy the GameObject and not the NetworkObject during DestroySceneNetworkObjects

* style

Added comment.

* fix

Some scenarios the NetworkObject and/or the GameObject can be null.
This update takes that into account.

* refactor fix

This is a cleaner and more accurate fix for the issue where a parent is destroyed before its child/children.  Added additional comments to explain why.

Author: NoelStephensUnity

com.unity.netcode.gameobjects/TestHelpers/Runtime/NetcodeIntegrationTest.cs

@@ -493,9 +493,22 @@ protected void DestroySceneNetworkObjects()
             var networkObjects = Object.FindObjectsOfType<NetworkObject>();
             foreach (var networkObject in networkObjects)
             {
+                // This can sometimes be null depending upon order of operations
+                // when dealing with parented NetworkObjects.  If NetworkObjectB
+                // is a child of NetworkObjectA and NetworkObjectA comes before
+                // NetworkObjectB in the list of NeworkObjects found, then when
+                // NetworkObjectA's GameObject is destroyed it will also destroy
+                // NetworkObjectB's GameObject which will destroy NetworkObjectB.
+                // If there is a null entry in the list, this is the most likely
+                // scenario and so we just skip over it.
+                if (networkObject == null)
+                {
+                    continue;
+                }
                 if (CanDestroyNetworkObject(networkObject))
                 {
-                    Object.DestroyImmediate(networkObject);
+                    // Destroy the GameObject that holds the NetworkObject component
+                    Object.DestroyImmediate(networkObject.gameObject);
                 }
             }
         }