flutter context become null if list is not empty - flutter

Question

I have a form to create/update activity which is a stateful widget like this:
class ActivityForm extends StatefulWidget {
final Activity activity;
final bool isEdit;
const ActivityForm({Key key, this.activity, this.isEdit}) : super(key: key);
#override
_ActivityFormState createState() => _ActivityFormState();
}
class _ActivityFormState extends State<ActivityForm> {
final _formKey = GlobalKey<FormState>();
List<int> activityDetailIdToBeDeleted;
...
void submitHandler() async {
setState(() {
_isLoading = true;
});
// Mapping input field to activity model
Activity activity = Activity(
id: widget.activity.id,
tanggal: _tanggalController.text,
uraianKegiatan: _uraianKegiatanController.text,
pic: _picController.text,
jumlahTim: int.parse(_jumlahTimController.text),
kendala: _kendalaController.text,
penyelesaian: _penyelesaianController.text,
approverUserId: selectedApprovalUser,
rincianPekerjaan: rincianPekerjaanList,
status: 'PENDING');
if (widget.isEdit) {
await Provider.of<ActivityProvider>(context, listen: false)
.updateActivity(activity, activityDetailIdToBeDeleted);
} else {
await Provider.of<ActivityProvider>(context, listen: false)
.createActivity(activity);
}
Navigator.pop(context);
}
in my form there are some detail list of activities. When i press delete it adds the id of
the activity detail to the list of "to be deleted" :
IconButton(
icon: Icon(Icons.delete),
onPressed: () => removeRincianPekerjaan(
activityDetail.description),),
here's the function:
void removeRincianPekerjaan(desc) {
if (widget.isEdit) {
int detailId = rincianPekerjaanList
.where((element) => element.description == desc)
.first
.id;
if (!activityDetailIdToBeDeleted.contains(detailId))
activityDetailIdToBeDeleted.add(detailId);
print(activityDetailIdToBeDeleted);
}
setState(() {
rincianPekerjaanList
.removeWhere((element) => element.description == desc);
});
}
that list of ids will be sent to provider which looks like this:
Future<bool> updateActivity(
Activity activity, List<int> activityDetailToBeDeleted) async {
final response = await ActivityService()
.updateActivity(activity.id.toString(), activity.toMap());
if (response != null) {
// Update Success.
if (response.statusCode == 200) {
var body = json.decode(response.body);
removeActivityToList(activity);
activity = Activity.fromMap(body['activity']);
addActivityToList(activity);
if (activityDetailToBeDeleted.isNotEmpty) {
print("to be deleted is not empty.");
print("ID to be deleted: ${activityDetailToBeDeleted.join(",")}");
final res = await ActivityService()
.deleteActivityDetail(activityDetailToBeDeleted.join(","));
if (res.statusCode == 204) {
print('ID DELETED.');
return true;
}
}
return true;
}
// Error unauthorized / token expired
else if (response.statusCode == 401) {
var reauth = await AuthProvider().refreshToken();
if (reauth) {
return await createActivity(activity);
} else {
return Future.error('unauthorized');
}
}
return Future.error('server error');
}
return Future.error('connection failed');
}
the problem is if the list is empty, or i didn't add the id to the list, builder context is not null but if i add an id to the list "to be deleted", the Navigator.pop(context) will throw an error because context is null. I don't understand why it becomes null.
full code in here

Answer 1

Your [context] must came from the build() method. Like this.
void submitHandler(BuildContext context) async {
setState(() {
_isLoading = true;
});
// Mapping input field to activity model
Activity activity = Activity(
id: widget.activity.id,
tanggal: _tanggalController.text,
uraianKegiatan: _uraianKegiatanController.text,
pic: _picController.text,
jumlahTim: int.parse(_jumlahTimController.text),
kendala: _kendalaController.text,
penyelesaian: _penyelesaianController.text,
approverUserId: selectedApprovalUser,
rincianPekerjaan: rincianPekerjaanList,
status: 'PENDING');
if (widget.isEdit) {
await Provider.of<ActivityProvider>(context, listen: false)
.updateActivity(activity, activityDetailIdToBeDeleted);
} else {
await Provider.of<ActivityProvider>(context, listen: false)
.createActivity(activity);
}
SchedulerBinding.instance.addPostFrameCallback((_) {
Navigator.pop(context);
});
}