fix: ServerDestroySpawnedSceneObjects throws an exception before switching scene (#618)

* fix: crash during scene transition

If you have in-scene placed NetworkObjects and you transition to a new scene then a crash will occur in NetworkSpawnManager.ServerDestroySpawnedSceneObjects due to NetworkObjects being removed from the SpawnedObjectsList during the "OnDestroy" invocation.  This fixes that crash.

* style: comments and standards

Adding some comments as to why the code is added, camelCase for locals, and generic Var as opposed to specific type for declaration.

* refactor: comments and better approach

This applies a better approach to the issue of removing objects from a list that is being iterated over without throwing the "Collection was modified; enumeration operation may not execute" exception due to NetworkObjects removing themselves from the SpawnedObjectList during OnDestroy.

* refactor: comment

Minor tweak to the comment as to why the code was added.

Author: NoelStephensUnity

com.unity.multiplayer.mlapi/Runtime/Spawning/NetworkSpawnManager.cs

@@ -541,19 +541,28 @@ internal static void ServerResetShudownStateForSceneObjects()
             }
         }
 
+        /// <summary>
+        /// Gets called only by NetworkSceneManager.SwitchScene
+        /// </summary>
         internal static void ServerDestroySpawnedSceneObjects()
         {
-            foreach (var sobj in SpawnedObjectsList)
+            //This Allocation is "ok" for now because this code only executes when a new scene is switched to
+            //We need to create a new copy the HashSet of NetworkObjects (SpawnedObjectsList) so we can remove
+            //objects from the HashSet (SpawnedObjectsList) without causing a list has been modified exception to occur.
+            var spawnedObjects = SpawnedObjectsList.ToList();
+            foreach (var sobj in spawnedObjects)
             {
                 if ((sobj.IsSceneObject != null && sobj.IsSceneObject == true) || sobj.DestroyWithScene)
                 {
                     if (CustomDestroyHandlers.ContainsKey(sobj.PrefabHash))
                     {
+                        SpawnedObjectsList.Remove(sobj);
                         CustomDestroyHandlers[sobj.PrefabHash](sobj);
                         OnDestroyObject(sobj.NetworkObjectId, false);
                     }
                     else
                     {
+                        SpawnedObjectsList.Remove(sobj);
                         MonoBehaviour.Destroy(sobj.gameObject);
                     }
                 }