fix: shutdown causes exception if invoked twice [MTT-6752] (#2622)

* fix

This fixes the issue with shutdown throwing an exception when being invoked again within the shutdown callback.

* test

Added test to validate the fix

Author: NoelStephensUnity

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -14,6 +14,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 ### Fixed
 
 - Fixed issue where `NetworkObject.SpawnWithObservers` was not being honored for late joining clients. (#2623)
+- Fixed issue where invoking `NetworkManager.Shutdown` multiple times, depending upon the timing, could cause an exception. (#2622)
 
 ## Changed
 

com.unity.netcode.gameobjects/Runtime/Core/NetworkManager.cs

@@ -942,10 +942,16 @@ public void Shutdown(bool discardMessageQueue = false)
             if (IsServer || IsClient)
             {
                 m_ShuttingDown = true;
-                MessageManager.StopProcessing = discardMessageQueue;
+                if (MessageManager != null)
+                {
+                    MessageManager.StopProcessing = discardMessageQueue;
+                }
             }
 
-            NetworkConfig.NetworkTransport.OnTransportEvent -= ConnectionManager.HandleNetworkEvent;
+            if (NetworkConfig != null && NetworkConfig.NetworkTransport != null)
+            {
+                NetworkConfig.NetworkTransport.OnTransportEvent -= ConnectionManager.HandleNetworkEvent;
+            }
         }
 
         // Ensures that the NetworkManager is cleaned up before OnDestroy is run on NetworkObjects and NetworkBehaviours when unloading a scene with a NetworkManager

testproject/Assets/Tests/Runtime/NetworkManagerTests.cs

@@ -4,6 +4,7 @@
 using Unity.Netcode.TestHelpers.Runtime;
 using UnityEngine;
 using UnityEngine.SceneManagement;
+using UnityEngine.TestTools;
 
 namespace TestProject.RuntimeTests
 {
@@ -94,5 +95,25 @@ public void ValidateHostSettings()
             Assert.IsTrue(m_NumberOfTimesInvoked == 1, $"OnClientConnectedCallback was invoked {m_NumberOfTimesInvoked} as opposed to just once!");
             Assert.IsTrue(m_NetworkBehaviourIsServerWasSet, $"IsServer was not true when OnClientConnectedCallback was invoked!");
         }
+
+        /// <summary>
+        /// Validate shutting down a second time does not cause an exception.
+        /// </summary>        
+        [UnityTest]
+        public IEnumerator ValidateShutdown()
+        {
+            // Register for the server stopped notification so we know we have shutdown completely
+            m_ServerNetworkManager.OnServerStopped += M_ServerNetworkManager_OnServerStopped;
+            // Shutdown
+            m_ServerNetworkManager.Shutdown();
+            yield return s_DefaultWaitForTick;
+        }
+
+        private void M_ServerNetworkManager_OnServerStopped(bool obj)
+        {
+            m_ServerNetworkManager.OnServerStopped -= M_ServerNetworkManager_OnServerStopped;
+            // Verify that we can invoke shutdown again without an exception
+            m_ServerNetworkManager.Shutdown();
+        }
     }
 }