NullReferenceException in UdpTransport ReceiveFromCallback method

[dimochka]

Checking for _socket is null and using _socket after this.
if( _socket == null )
_asyncCallback(AsyncRequestResult.Terminated, new IPEndPoint(_socket.AddressFamily == AddressFamily.InterNetwork ? IPAddress.Any : IPAddress.IPv6Any, 0), null, 0);
else
_asyncCallback(AsyncRequestResult.Terminated, new IPEndPoint(IPAddress.Any, 0), null, 0);
return; // socket has been closed. no new operations are possible.