Failed Assertion: Line 20 Pos 16: 'longitude != Null': Is Not True

I'm trying to build a TaxiApp where a passenger is supposed to search for a dropOff location but each time the user searches for a location and clicks on it using the Google Autocomplete It keeps bringing this error: [ERROR:flutter/lib/ui/ui_dart_state.cc(199)] Unhandled Exception: 'package:google_maps_flutter_platform_interface/src/types/location.dart': Failed assertion: line 20 pos 16: 'longitude != null': is not true.

That error points to this method

Future<void> getPlaceDirection() async
  {
    var initialPos = Provider.of<AppData>(context, listen: false).pickUpLocation;
    var finalPos = Provider.of<AppData>(context, listen: false).dropOffLocation;

    var pickUpLatLng = LatLng(initialPos.latitude, initialPos.longitude);
    var dropOffLatLng = LatLng(finalPos.latitude, finalPos.longitude);

    showDialog(
      context: context, 
      builder: (BuildContext content) => ProgressDialog(message: "Please wait...",)
    );
    var details = await AssistantMethods.obtainPlaceDirectionDetails(pickUpLatLng, dropOffLatLng);
    setState(() {
          tripDirectionDetails = details;
        });

    Navigator.pop(context);

    print("This is the encoded points:: ");
    print(details.encodedPoints);

    PolylinePoints polylinePoints = PolylinePoints();
    List<PointLatLng> decodedPolyLinePointsResult = polylinePoints.decodePolyline(details.encodedPoints);
    pLineCoordinates.clear();

    if (decodedPolyLinePointsResult.isNotEmpty)
    {
      decodedPolyLinePointsResult.forEach((PointLatLng pointLatLng)
      {
         pLineCoordinates.add(LatLng(pointLatLng.latitude, pointLatLng.longitude));
      });
    }

    polylineSet.clear();

    setState(() {
     Polyline polyline = Polyline(
      color: orange,
      polylineId: PolylineId("PolylineID"),
      jointType: JointType.round,
      points: pLineCoordinates,
      width: 5,
      startCap: Cap.roundCap,
      endCap: Cap.roundCap,
      geodesic: true,
       );

       polylineSet.add(polyline);
    });

    LatLngBounds latLngBounds;
      if(pickUpLatLng.latitude > dropOffLatLng.latitude && pickUpLatLng.longitude > dropOffLatLng.longitude)
      {
        latLngBounds = LatLngBounds(southwest: dropOffLatLng, northeast: pickUpLatLng);
      }
      else if(pickUpLatLng.longitude> dropOffLatLng.longitude)
      {
        latLngBounds = LatLngBounds(southwest: LatLng(pickUpLatLng.latitude, dropOffLatLng.longitude), 
        northeast: LatLng(dropOffLatLng.latitude, pickUpLatLng.longitude));
      }
      else if(pickUpLatLng.latitude > dropOffLatLng.latitude)
      {
        latLngBounds = LatLngBounds(southwest: LatLng(dropOffLatLng.latitude, pickUpLatLng.longitude), 
        northeast: LatLng(pickUpLatLng.latitude, dropOffLatLng.longitude));
      } 
      else 
      {
        latLngBounds = LatLngBounds(southwest: pickUpLatLng, northeast: dropOffLatLng);
      }

      newGoogleMapController.animateCamera(CameraUpdate.newLatLngBounds(latLngBounds, 70));

      Marker pickUpLocMarker = Marker(
        icon: BitmapDescriptor.defaultMarkerWithHue(BitmapDescriptor.hueBlue),
        infoWindow: InfoWindow(title: initialPos.placeName, snippet: "My location"),
        position:  pickUpLatLng,
        markerId: MarkerId("pickUpId"),
      );

      Marker dropOffLocMarker = Marker(
        icon: BitmapDescriptor.defaultMarkerWithHue(BitmapDescriptor.hueRed),
        infoWindow: InfoWindow(title: finalPos.placeName, snippet: "Drop-off location"),
        position:  dropOffLatLng,
        markerId: MarkerId("dropOffId"),
      );

      setState(() {
              markerSet.add(pickUpLocMarker);
              markerSet.add(dropOffLocMarker);
            });

      Circle pickUpLocCircle = Circle(
        fillColor: Colors.blue,
        center: pickUpLatLng,
        radius: 12,
        strokeWidth: 4,
        strokeColor: Colors.blueAccent,
        circleId: CircleId("pickUpId")
      );

      Circle dropOffLocCircle = Circle(
        fillColor: Colors.red,
        center: dropOffLatLng,
        radius: 12,
        strokeWidth: 4,
        strokeColor: Colors.redAccent,
        circleId: CircleId("dropOffId"),
      ); 

      setState(() {
              circleSet.add(pickUpLocCircle);
              circleSet.add(dropOffLocCircle);
            }); 
  } 

Understanding The Problem

You need to use FutureBuilder widget because your App depends on on waiting for another operation to complete. Otherwise latitude value will be NULL.

Check this - https://api.flutter.dev/flutter/widgets/FutureBuilder-class.html

Solution

You must wrap your body of Scaffold (where you are actually calling the main map widget) with FutureBuilder. You haven't added the main function from where you are calling your map widget but the following is the pseudo-code to do it -

 @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        iconTheme: IconThemeData(color: Colors.white),
        title: Text('Map'),
        centerTitle: true,
        backgroundColor: Colors.blue,
      ),
     //this mapWidget should be wrapped in future builder
      body: mapWidget(),
    );
  }

 Widget mapWidget() {
    return FutureBuilder(
                
                future: getPlaceDirection,//adding your main location widget in future
                builder: (context, state) {
                  if (state.connectionState == ConnectionState.active ||
                      state.connectionState == ConnectionState.waiting) {
                      return SpinKitRipple(
                        itemBuilder: (BuildContext context, int index) {
                          return DecoratedBox(
                               decoration: BoxDecoration(
                               color: index.isEven ? Colors.grey : 
                                    Color(0xffffb838),
                               ),);},);
                  } 
                  else {

//here you should write your main logic to be shown once latitude ! = NULL
               
                   }
}