I am using [background_locator_2][1] plugin, However when I run it with some modification I get this error

[ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: type 'Null' is not a subtype of type 'LocationDto'

This is the code i am using

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  ReceivePort port = ReceivePort();

  String logStr = '';
  bool? isRunning;
  LocationDto? lastLocation;

  @override
  void initState() {
    super.initState();

    if (IsolateNameServer.lookupPortByName(
            LocationServiceRepository.isolateName) !=
        null) {
      IsolateNameServer.removePortNameMapping(
          LocationServiceRepository.isolateName);
    }

    IsolateNameServer.registerPortWithName(
        port.sendPort, LocationServiceRepository.isolateName);

    port.listen(
      (dynamic data) async {
        await updateUI(data);
      },
    );
    initPlatformState();
  }

  @override
  void dispose() {
    super.dispose();
  }

  Future<void> updateUI(LocationDto data) async {
    final log = await FileManager.readLogFile();

    await _updateNotificationText(data);

    setState(() {
      lastLocation = data;
      logStr = log;
    });
  }

  Future<void> _updateNotificationText(LocationDto data) async {
    if (data == null) {
      return;
    }

    await BackgroundLocator.updateNotificationText(
        title: "new location received",
        msg: "${DateTime.now()}",
        bigMsg: "${data.latitude}, ${data.longitude}");
  }

  Future<void> initPlatformState() async {
    print('Initializing...');
    await BackgroundLocator.initialize();
    logStr = await FileManager.readLogFile();
    print('Initialization done');
    final _isRunning = await BackgroundLocator.isServiceRunning();
    setState(() {
      isRunning = _isRunning;
    });
    print('Running ${isRunning.toString()}');
  }

  @override
  Widget build(BuildContext context) {
    final start = SizedBox(
      width: double.maxFinite,
      child: ElevatedButton(
        child: Text('Start'),
        onPressed: () {
          _onStart();
        },
      ),
    );
    final stop = SizedBox(
      width: double.maxFinite,
      child: ElevatedButton(
        child: Text('Stop'),
        onPressed: () {
          onStop();
        },
      ),
    );
    final clear = SizedBox(
      width: double.maxFinite,
      child: ElevatedButton(
        child: Text('Clear Log'),
        onPressed: () {
          FileManager.clearLogFile();
          setState(() {
            logStr = '';
          });
        },
      ),
    );
    String msgStatus = "-";
    if (isRunning != null) {
      if (isRunning!) {
        msgStatus = 'Is running';
      } else {
        msgStatus = 'Is not running';
      }
    }
    final status = Text("Status: $msgStatus");

    final log = Text(
      logStr,
    );

    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Flutter background Locator'),
        ),
        body: Container(
          width: double.maxFinite,
          padding: const EdgeInsets.all(22),
          child: SingleChildScrollView(
            child: Column(
              crossAxisAlignment: CrossAxisAlignment.center,
              children: <Widget>[start, stop, clear, status, log],
            ),
          ),
        ),
      ),
    );
  }

  void onStop() async {
    await BackgroundLocator.unRegisterLocationUpdate();
    final _isRunning = await BackgroundLocator.isServiceRunning();
    setState(() {
      isRunning = _isRunning;
    });
  }

  void _onStart() async {
    //if (await isLocationAlwaysGranted()) {
    await _startLocator();
    final _isRunning = await BackgroundLocator.isServiceRunning();

    setState(() {
      isRunning = _isRunning;
      lastLocation = null;
    });
    // } else {
    // show error
  }
}

Future<bool> isLocationAlwaysGranted() async =>
    await Permission.locationAlways.isGranted;

/// Tries to ask for "location always" permissions from the user.
/// Returns `true` if successful, `false` othervise.
Future<bool> askForLocationAlwaysPermission() async {
  bool granted = await Permission.locationAlways.isGranted;

  if (!granted) {
    granted =
        await Permission.locationAlways.request() == PermissionStatus.granted;
  }

  return granted;
}

Future<void> _startLocator() async {
  Map<String, dynamic> data = {'countInit': 1};
  return await BackgroundLocator.registerLocationUpdate(
      LocationCallbackHandler.callback,
      initCallback: LocationCallbackHandler.initCallback,
      initDataCallback: data,
      disposeCallback: LocationCallbackHandler.disposeCallback,
      iosSettings: IOSSettings(
          accuracy: LocationAccuracy.NAVIGATION,
          distanceFilter: 0,
          stopWithTerminate: true),
      autoStop: false,
      androidSettings: AndroidSettings(
          accuracy: LocationAccuracy.NAVIGATION,
          interval: 5,
          distanceFilter: 0,
          client: LocationClient.google,
          androidNotificationSettings: AndroidNotificationSettings(
              notificationChannelName: 'Location tracking',
              notificationTitle: 'Start Location Tracking',
              notificationMsg: 'Track location in background',
              notificationBigMsg:
                  'Background location is on to keep the app up-tp-date with your location. This is required for main features to work properly when the app is not running.',
              notificationIconColor: Colors.grey,
              notificationTapCallback:
                  LocationCallbackHandler.notificationCallback)));
}

The error is in this line under initState when I start or stop the plugin.

  port.listen(
      (dynamic data) async {
        await updateUI(data);
      },
    );

The original code didn't have null safety so i tried to modify it. However it is pretty evident my knowledge is limited. [1]: https://pub.dev/packages/background_locator_2

CodePudding user response：

the data you are listening to from the port (dynamic data) can be null and you are sending it to the function updateUI which does not accept nullable type.

you can either check if the data is not null before calling await updateUI(data); or you can make the function updateUI accepts null, i.e (Future<void> updateUI(LocationDto? data) async {) and handle the case that the data is nullable inside the function