Fixed null ref on shutdown

Author: TwoTenPvP

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

@@ -696,10 +696,13 @@ public void Shutdown()
             IsClient = false;
             NetworkConfig.NetworkTransport.OnTransportEvent -= HandleRawTransportPoll;
 
-            SpawnManager.DestroyNonSceneObjects();
-            SpawnManager.ServerResetShudownStateForSceneObjects();
+            if (SpawnManager != null)
+            {
+                SpawnManager.DestroyNonSceneObjects();
+                SpawnManager.ServerResetShudownStateForSceneObjects();
 
-            SpawnManager = null;
+                SpawnManager = null;
+            }
 
             //The Transport is set during Init time, thus it is possible for the Transport to be null
             NetworkConfig?.NetworkTransport?.Shutdown();