v2.6.4 fix socket null exception

Author: psygames

Assets/UnityWebSocket/Scripts/Runtime/Implementation/NoWebGL/WebSocket.cs

@@ -42,7 +42,7 @@ public WebSocketState ReadyState
         public event EventHandler<MessageEventArgs> OnMessage;
 
         private ClientWebSocket socket;
-        private bool isOpening => socket.State == System.Net.WebSockets.WebSocketState.Open;
+        private bool isOpening => socket != null && socket.State == System.Net.WebSockets.WebSocketState.Open;
 
         #region APIs
         public WebSocket(string address)