包裹你的逻辑在确定的验证码为空声明，在调用它的属性。如果是，则通过发送消息到客户端，让他们知道他们超过超时而他们需要再次尝试处理错误。

Wrap your logic in a statement that determines if the Captcha is null, prior to calling it's properties. If it is, handle the error by sending a message to the client letting them known they exceeded the timeout and they need try again.