fix null reference exception in web socket transport when shutting down a host (#206)

Author: BigSpaceships

Transports/com.community.netcode.transport.websocket/Runtime/WebSocketTransport.cs

@@ -24,6 +24,7 @@ public class WebSocketTransport : NetworkTransport
 
         public override void DisconnectLocalClient()
         {
+            if (WebSocketClient == null) return;
             WebSocketClient.Close();
         }