connect method Null safety

Implementation

Future<void> connect(String url) async {
  if (url.length < 10) {
    logger.e('Socket url is too short');
    throw WebSocketError(message: 'Invalid URL supplied');
  }
  // already conncted on same url
  if (_connected && url == _url) return;

  var hdrs = <String, dynamic>{};
  hdrs['Content-Type'] = 'application/json;charset=utf8';

  try {
    _socket = WebSocketChannel.connect(Uri.parse(url));
    _disconnect = false;
    _connected = true;
    _url = url;

    _startPing();

    _functionConnect();
  } catch (e) {
    logger.e('Websocket connection error: ' + e.toString());
    throw WebSocketError(message: e.toString());
  }

  _rx = _socket!.stream;

  _subscription = _rx?.listen(
    onReceiveData,
    onError: onReceiveError,
    onDone: onReceiveDone,
    cancelOnError: true,
  );
}