fix: Empty prefab removal (#919)

This fixes a bug where a list was being removed from while iterated over.

`networkPrefabIndexToRemove` contains indices of prefabs to remove in ascending order so we need to remove them in reverse order or else the indices all shift and we can an argument exception.

Author: jlao

com.unity.multiplayer.mlapi/Runtime/Core/NetworkManager.cs

@@ -496,9 +496,10 @@ private void Initialize(bool server)
             }
 
             // Clear out anything that is invalid or not used (for invalid entries we already logged warnings to the user earlier)
-            foreach (var networkPrefabIndexToRemove in removeEmptyPrefabs)
+            // Iterate backwards so indices don't shift as we remove
+            for (int i = removeEmptyPrefabs.Count-1; i >= 0; i--)
             {
-                NetworkConfig.NetworkPrefabs.RemoveAt(networkPrefabIndexToRemove);
+                NetworkConfig.NetworkPrefabs.RemoveAt(removeEmptyPrefabs[i]);
             }
             removeEmptyPrefabs.Clear();