The method 'getUserData' was called on null. Receiver: null Tried calling: getUserData() - flutter

Question

I'm trying to get the user's currency from my server. Until now I'm able to do that but the problem is the data keeps rebuild itself everytime the app gets rebuilt. I tried to fix that by this way :
Future<dynamic> userDataFuture;
ApiService apiService;
#override
void initState() {
userDataFuture = apiService.getUserData();
super.initState();
}
but fails as it gives me an error saying :
The method 'getUserData' was called on null. Receiver: null Tried calling: getUserData()
this is my ApiService Class
Future getUserData() async {
String token = await AuthProvider().getToken();
String userID = await AuthProvider().getUserId();
final response = await Dio().get(
'$apiUrl/$userID',
options: Options(headers: {
'Content-Type': 'application/json',
'Accept': 'application/json',
'Authorization': 'Bearer $token',
}),
);
if (response.statusCode == 200) {
return response.data;
} else {
throw Exception('Failed to load data');
}
}
this is my UI where I want to show the user's currency:
child: Center(
child: FutureBuilder(
future: userDataFuture,
builder:
(BuildContext context, AsyncSnapshot snapshot) {
if (snapshot.hasError) {
return Center(
child: Text(
"Something wrong with message: ${snapshot.error.toString()}"),
);
} else if (snapshot.connectionState ==
ConnectionState.done) {
return Text(
snapshot.data[
'currency'], // This Should Change Depending in Settings of the User
style: TextStyle(
fontSize:
(device.localWidth * .1) * .43,
fontWeight: FontWeight.w500,
color: kLightTextColor));
} else {
return Center(
child: CircularProgressIndicator(),
);
}
},
// ),
),
),

Answer 1

You're calling getUserData() on an ApiService object that is not initialized -- ie. null.
Instead of this
ApiService apiService;
Actually initialize it like this
final apiService = ApiService();
That will get rid of your null error. You can also just pass apiService.getUserData() into your FutureBuilder and get rid of the userDataFuture object altogether.

Answer 2

first of all you have to use await, here like this
userDataFuture = await apiService.getUserData();
second you have to initialize you'r apiService variable like this:
ApiService apiService = new ApiService();
third, change you'r getUserData to return some data, it's void function.
fourth one you have to use somethink like this:
setState(() {
userDateFuture = await apiService.getUserData();
});
i hope, i was able to help.