Why I got : Unhandled Exception: 'package:flutter/src/material/scaffold.dart

I'm new with flutter and I want to connect devices with bluetooth. Actually the scan is workin very well but for connecting devices an exception was occured. The exception is : Unhandled Exception: 'package:flutter/src/material/scaffold.dart': Failed assertion: line 269 pos 7: '_scaffolds.isNotEmpty': ScaffoldMessenger.showSnackBar was called, but there are currently no descendant Scaffolds to present to. E/flutter ( 3688): #0 _AssertionErr

this is the code:

// Method to connect to bluetooth
  void _connect() async {
    setState(() {
      _isButtonUnavailable = true;
    });
    if (_device == null) {
      show('No device selected');
    } else {
      if (!isConnected) {
        await BluetoothConnection.toAddress(_device.address)
            .then((_connection) {
          print('Connected to the device');
          connection = _connection;
          setState(() {
            _connected = true;
          });

          connection.input.listen(null).onDone(() {
            if (isDisconnecting) {
              print('Disconnecting locally!');
            } else {
              print('Disconnected remotely!');
            }
            if (this.mounted) {
              setState(() {});
            }
          });
        }).catchError((error) {
          print('Cannot connect, exception occurred');
          print(error);
        });
        show('Device connected');

        setState(() => _isButtonUnavailable = false);
      }
    }
  }

  void onDataReceived(Uint8List data) {
    //   // Allocate buffer for parsed data
    int backspacesCounter = 0;
    data.forEach((byte) {
      if (byte == 8 || byte == 127) {
        backspacesCounter++;
      }
    });
    Uint8List buffer = Uint8List(data.length - backspacesCounter);
    int bufferIndex = buffer.length;

    // Apply backspace control character
    backspacesCounter = 0;
    for (int i = data.length - 1; i >= 0; i--) {
      if (data[i] == 8 || data[i] == 127) {
        backspacesCounter++;
      } else {
        if (backspacesCounter > 0) {
          backspacesCounter--;
        } else {
          buffer[--bufferIndex] = data[i];
        }
      }
    }
  }

  // Method to disconnect bluetooth
  void _disconnect() async {
    setState(() {
      _isButtonUnavailable = true;
      _deviceState = 0;
    });

    await connection.close();
    show('Device disconnected');
    if (!connection.isConnected) {
      setState(() {
        _connected = false;
        _isButtonUnavailable = false;
      });
    }
  }

  // Method to send message,
  // for turning the Bluetooth device on
  void _sendOnMessageToBluetooth() async {
    connection.output.add(utf8.encode("1" + "\r\n"));
    await connection.output.allSent;
    show('Device Turned On');
    setState(() {
      _deviceState = 1; // device on
    });
  }

  // Method to send message,
  // for turning the Bluetooth device off
  void _sendOffMessageToBluetooth() async {
    connection.output.add(utf8.encode("0" + "\r\n"));
    await connection.output.allSent;
    show('Device Turned Off');
    setState(() {
      _deviceState = -1; // device off
    });
  }

  // Method to show a Snackbar,
  // taking message as the text
  Future show(
    String message, {
    Duration duration: const Duration(seconds: 3),
  }) async {
    await new Future.delayed(new Duration(milliseconds: 100));
    ScaffoldMessenger.of(context).showSnackBar(
      new SnackBar(
        content: new Text(
          message,
        ),
        duration: duration,
      ),
    );
  }
}
I would be very thankful if you help me.