How to solve the unstable polyline in map flutter

Question

I want to draw the location distance on the map. Then when the user moves the location pin will also move it. I have refer this page. The problem I face is the polyline some time have come out some time did not come out. The packages that I use is google_maps_flutter: ^2.0.3, lutter_polyline_points: ^0.2.6, and location: ^4.1.1. Anyone can help me? Thanks in advance. Here is my code:

const double CAMERA_ZOOM = 18;
const double CAMERA_TILT = 80;
const double CAMERA_BEARING = 30;
const LatLng SOURCE_LOCATION = LatLng(2.3896, 122.2501);
const LatLng DEST_LOCATION = LatLng(-2.2855, 122.2625);

class MapPage extends StatefulWidget {
  @override
  _MapPageState createState() => _MapPageState();
}

class _MapPageState extends State<MapPage> {
  Completer<GoogleMapController> _controller = Completer();
  Set<Marker> _markers = Set<Marker>();

  Set<Polyline> _polylines = Set<Polyline>();
  List<LatLng> polylineCoordinates = [];
  PolylinePoints polylinePoints;

  String googleAPIKey = 'MY GOOGLE API KEY';

  BitmapDescriptor sourceIcon;
  BitmapDescriptor destinationIcon;

  LocationData currentLocation;

  LocationData destinationLocation;

  Location location;

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

    location = new Location();
    polylinePoints = PolylinePoints();

    location.onLocationChanged.listen((LocationData cLoc) {
      currentLocation = cLoc;
      updatePinOnMap();
    });

    setSourceAndDestinationIcons();

    setInitialLocation();
  }

  void setSourceAndDestinationIcons() async {
    sourceIcon = await BitmapDescriptor.fromAssetImage(
        ImageConfiguration(devicePixelRatio: 2.5), 'assets/driving_pin.png');

    destinationIcon = await BitmapDescriptor.fromAssetImage(
        ImageConfiguration(devicePixelRatio: 2.5),
        'assets/destination_map_marker.png');
  }

  void setInitialLocation() async {
    currentLocation = await location.getLocation();

    destinationLocation = LocationData.fromMap({
      "latitude": DEST_LOCATION.latitude,
      "longitude": DEST_LOCATION.longitude,
    });
  }

  @override
  Widget build(BuildContext context) {
    CameraPosition initialCameraPosition = CameraPosition(
        zoom: CAMERA_ZOOM,
        tilt: CAMERA_TILT,
        bearing: CAMERA_BEARING,
        target: SOURCE_LOCATION);

    if (currentLocation != null) {
      initialCameraPosition = CameraPosition(
          target: LatLng(currentLocation.latitude, currentLocation.longitude),
          zoom: CAMERA_ZOOM,
          tilt: CAMERA_TILT,
          bearing: CAMERA_BEARING);
    }
    return Scaffold(
      body: Stack(
        children: <Widget>[
          GoogleMap(
            initialCameraPosition: initialCameraPosition,
            myLocationEnabled: true,
            compassEnabled: true,
            tiltGesturesEnabled: false,
            markers: _markers,
            polylines: _polylines,
            mapType: MapType.normal,
            onMapCreated: (GoogleMapController controller) {
              _controller.complete(controller);
              showPinsOnMap();
            },
          )
        ],
      ),
    );
  }

  void showPinsOnMap() {
    //get a LatLng for the source location from the LocationData currentLocation object
    var pinPosition =
        LatLng(currentLocation.latitude, currentLocation.longitude);

    var destPosition =
        LatLng(destinationLocation.latitude, destinationLocation.longitude);

    _markers.add(Marker(
        markerId: MarkerId('sourcePin'),
        position: pinPosition,
        icon: sourceIcon));
    _markers.add(Marker(
        markerId: MarkerId('destPin'),
        position: destPosition,
        icon: destinationIcon));

    setPolylines();
  }

  void setPolylines() async {
    PolylineResult result = await polylinePoints.getRouteBetweenCoordinates(
        googleAPIKey,
        PointLatLng(currentLocation.latitude, currentLocation.longitude),
        PointLatLng(
            destinationLocation.latitude, destinationLocation.longitude));

    if (result.points.isNotEmpty) {
      result.points.forEach((PointLatLng point) {
        polylineCoordinates.add(LatLng(point.latitude, point.longitude));
      });
      setState(() {
        _polylines.add(Polyline(
            width: 5, 
            polylineId: PolylineId("poly"),
            color: Color.fromARGB(255, 40, 122, 198),
            points: polylineCoordinates));
      });
    }
  }

  void updatePinOnMap() async {
    CameraPosition cameraPosition = CameraPosition(
      zoom: CAMERA_ZOOM,
      tilt: CAMERA_TILT,
      bearing: CAMERA_BEARING,
      target: LatLng(currentLocation.latitude, currentLocation.longitude),
    );

    final GoogleMapController controller = await _controller.future;
    controller.animateCamera(CameraUpdate.newCameraPosition(cameraPosition));

    setState(() {
      var pinPosition =
          LatLng(currentLocation.latitude, currentLocation.longitude);

      _markers.add(Marker(
          markerId: MarkerId('sourcePin'),
          position: pinPosition, 
          icon: sourceIcon));
    });
  }
}