Android Studio – Flutter build method is being called before the variables are fully initialized which is causing app crashes

I want to initialize the value of _lastMapPosition from the session variables but In my case the builds method runs before intistate method. Is there a simple way to initialize it before build widget method? I am new to flutter kindly provide complete details. I have tried futurebuilder but i think I tried it all wrong.

import 'dart:async';
    import 'package:flutter/material.dart';
    import 'package:google_maps_flutter/google_maps_flutter.dart';
    import 'package:permission/permission.dart';
    import 'package:flutter_session/flutter_session.dart';
    class GmyApp extends StatefulWidget {
      const GmyApp({Key key}) : super(key: key);
      @override
      _GmyApp createState() => _GmyApp();
    }
    class _GmyApp extends State<GmyApp> {
      var latitude_data;
      var longitude_data;
      Completer<GoogleMapController> _controller = Completer();
      static const LatLng _center = const LatLng(10.8505, 76.2711);
      final Set<Marker> _markers = {};
      LatLng _lastMapPosition= _center;
      MapType _currentMapType = MapType.normal;
      void _onMapTypeButtonPressed() {
        setState(() {
          _currentMapType = _currentMapType == MapType.normal
              ? MapType.satellite
              : MapType.normal;
        });
      }
      void _onAddMarkerButtonPressed() {
        setState(() {
          _markers.add(Marker(
            // This marker id can be anything that uniquely identifies each marker.
            markerId: MarkerId(_lastMapPosition.toString()),
            position: _lastMapPosition,
            infoWindow: InfoWindow(
              title: 'Your last location',
              snippet: 'Location Refreshed every 1 hour',
            ),
            icon: BitmapDescriptor.defaultMarker,
          ));
        });
      }
      void _onCameraMove(CameraPosition position) {
        _lastMapPosition = position.target;
      }
      void _onMapCreated(GoogleMapController controller) {
        _controller.complete(controller);
      }
      @override
      void initState() {
        // TODO: implement initState
        super.initState();
        setGps();
      }
      Future setGps() async {
        bool visible = false;
        print('I am in GPS setter*');
        // var session = FlutterSession();
        latitude_data = await FlutterSession().get('lat');
        longitude_data = await FlutterSession().get('longi');
        print('User logi Id is in GPS* $longitude_data');
        print('User lat is in GPS* $latitude_data');
        LatLng center1=LatLng(latitude_data, longitude_data);
        print('Lat .lat is in GPS  $center1');
         _lastMapPosition = center1;
        print('Lat .lat lastMap Position is  $_lastMapPosition');
      }
      @override
      Widget build(BuildContext context) {
        return MaterialApp(
          home: Scaffold(
            appBar: AppBar(
              title: Text('Your Current Location is'),
              backgroundColor: Colors.green[700],
            ),
            body: Stack(
              children: <Widget>[
                GoogleMap(
                  onMapCreated: _onMapCreated,
                  initialCameraPosition: CameraPosition(
                    target: _lastMapPosition,
                    zoom: 11.0,
                  ),
                  mapType: _currentMapType,
                  markers: _markers,
                  onCameraMove: _onCameraMove,
                ),
                Padding(
                  padding: const EdgeInsets.all(16.0),
                  child: Align(
                    alignment: Alignment.topRight,
                    child: Column(
                      children: <Widget> [
                        FloatingActionButton(
                          onPressed: _onMapTypeButtonPressed,
                          heroTag: "btn1",
                          materialTapTargetSize: MaterialTapTargetSize.padded,
                          backgroundColor: Colors.green,
                          child: const Icon(Icons.map, size: 36.0),
                        ),
                        SizedBox(height: 16.0),
                        FloatingActionButton(
                          onPressed: _onAddMarkerButtonPressed,
                          heroTag: "btn2",
                          materialTapTargetSize: MaterialTapTargetSize.padded,
                          backgroundColor: Colors.green,
                          child: const Icon(Icons.add_location, size: 36.0),
                        ),
                      ],
                    ),
                  ),
                ),
              ],
            ),
          ),
        );
      }
    }