tried to fix null reference exception in websocket server middleware

Author: kerryjiang

src/SuperSocket.WebSocket.Server/WebSocketServerMiddleware.cs

@@ -62,12 +62,18 @@ public override void Start(IServer server)
 
         public override void Shutdown(IServer server)
         {
+            _sessionContainerMiddleware = null;
+
             var checkTimer = _checkingTimer;
-            _checkingTimer = null;
-            checkTimer.Change(Timeout.Infinite, Timeout.Infinite);
-            checkTimer.Dispose();
 
-            _sessionContainerMiddleware = null;
+            if (checkTimer == null)
+                return;
+
+            if (Interlocked.CompareExchange(ref _checkingTimer, null, checkTimer) == checkTimer)
+            {
+                checkTimer.Change(Timeout.Infinite, Timeout.Infinite);
+                checkTimer.Dispose();
+            }            
         }
 
         public override ValueTask<bool> RegisterSession(IAppSession session)