Provider cannot get value - flutter

Question

i am building mobile app using flutter. i try to use provider but it doesn't get the value.
this is the code for class change notifier
class StoreProvider with ChangeNotifier{
UserServices _userServices = UserServices();
StoreServices _storeServices = StoreServices();
User user = FirebaseAuth.instance.currentUser;
String userLocation = "";
String selectedStore;
String selectedStoreId;
getSelectedStore(storeName, storeId){
this.selectedStore = storeName;
this.selectedStoreId = storeId;
notifyListeners();
}
}
this is the code where i call this function
ListView(
shrinkWrap: true,
scrollDirection: Axis.horizontal,
children: snapshot.data.docs.map((DocumentSnapshot document) {
return InkWell(
onTap: (){
_storeData.getSelectedStore(document['shopName'], document['uid']);
pushNewScreenWithRouteSettings(
context,
settings: RouteSettings(name: VendorHomeScreen.id),
screen: VendorHomeScreen(),
withNavBar: true,
pageTransitionAnimation: PageTransitionAnimation.cupertino,
);
},
child: Padding(
padding: const EdgeInsets.only(top: 4, right: 4, left: 4),
child: Container(
width: 120,
child: Column(
children: [
SizedBox(
width: 120,
height: 100,
child: Card(
child: ClipRRect(
borderRadius: BorderRadius.circular(4),
child: Image.network(document['imageUrl'],))),
),
Container(
height: 35,
child: Text(document['shopName'], style: TextStyle(
fontSize: 14, fontWeight: FontWeight.bold,
), maxLines: 2, overflow: TextOverflow.ellipsis,),
),
],
),
),
),
);
}).toList(),
),
this is the code where i want to get the value that i put to the provider
var _store = Provider.of<StoreProvider>(context);
return Scaffold(
body: NestedScrollView(
headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
return [
SliverAppBar(
iconTheme: IconThemeData(
color: Colors.white,
),
actions: [
IconButton(
onPressed: () {},
icon: Icon(CupertinoIcons.search),
)
],
title: Text(
**//this is the error**
**//this is the error**
**//this is the error**
**//this is the error**
**//this is the error**
**_store.selectedStoreId,**
style:
TextStyle(fontWeight: FontWeight.bold, color: Colors.white),
),
)
];
},
body: Center(
child: Text('vendor screen'),
),
));
and this is the main file
void main() async {
Provider.debugCheckInvalidValueType = null;
WidgetsFlutterBinding.ensureInitialized();
await Firebase.initializeApp();
runApp(MultiProvider(
providers: [
ChangeNotifierProvider(create: (_) => AuthProvider()),
ChangeNotifierProvider(create: (_) => StoreProvider()),
],
child: MyApp(),
));
}
class MyApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
debugShowCheckedModeBanner: false,
theme: ThemeData(primaryColor: Colors.lightBlueAccent, fontFamily: 'Lato'),
initialRoute: SplashScreen.id,
routes: {
SplashScreen.id: (context) => SplashScreen(),
LoginPage.id: (context) => LoginPage(),
SignupPage.id: (context) => SignupPage(),
HomePage.id: (context) => HomePage(),
ResetPassword.id: (context) => ResetPassword(),
MainScreen.id: (context)=> MainScreen(),
VendorHomeScreen.id: (context) => VendorHomeScreen(),
},
);
}
}
i always get 'a non null A non-null String must be provided to a Text widget.'
i think the problem is when i put the value in the change notifier class and i try to get the value in other class, the value is null.
does anyone know what is the mistake??
thank you