Shared Preferences set null after setting a new value to the key - flutter

Question

Am working on the task of changing the user name, however, when I click the button of changing the firstName and lastName, the shared preference is not set. This is my API Provider class am hitting :
class ChangeNameApiProvider {
Future<SharedPreferences> _pref = SharedPreferences.getInstance();
static const String USER = "USER";
// ignore: missing_return
Future<int> changeName(
String firstName, String lastName, String userId) async {
SharedPreferences prefs = await _pref;
String user = prefs.getString(USER);
var _user = jsonDecode(user);
final response = await http.put(
'${BaseUrl.BASE_URL}/v1/Users/$userId',
headers: <String, String>{
'Content-Type': 'application/json; charset=UTF-8',
'Authorization': 'Bearer ${_user['token']}'
},
body: jsonEncode(<String, String>{
'first_name': firstName,
'last_name': lastName,
}),
);
if (response.statusCode == 200) {
var data = jsonDecode(response.body);
_user['user'] = data;
_pref.then((SharedPreferences sharedPref) {
prefs = sharedPref;
print('after output $_user');
prefs.setString(USER, _user.toString());
print('after 2 output $_user');
});
return response.statusCode;
} else {
return response.statusCode;
}
}
}
Then, this is the Button on the form I trigger to call the above changeName() method :
FlatButton(
// ignore: sdk_version_ui_as_code
onPressed: () async => {
if (_formKey.currentState.validate())
{
changeNameRepository
.editName(_firstNameController.text,
_lastNameController.text, '${widget.userId}')
.then((value) async {
if (value == 200) {
_showToastMessage("Name changed successfully.");
await Navigator.pop(context);
}
})
}
},
child: Text(
'SAVE CHANGES',
textAlign: TextAlign.right,
style: TextStyle(
color: Color(0xFFD2232A),
letterSpacing: 1.25,
fontSize: 18,
fontWeight: FontWeight.w500,
),
),
)
Then, this is the repository :
class ChangeNameRepository {
final changeNameApiProvider = ChangeNameApiProvider();
Future editName(firstName, lastName, userId) =>
changeNameApiProvider.changeName(firstName, lastName, userId);
}
So, when all the above code is executed, it's supposed to update the shared preferences, then pop back to the previous screen with updated Shared Preference vale USER.
But it seems, the value is set to null, it's not updating.
Finally, let me share how I retrieve the shared preferences on the first page after popping from the changing nanme page:
_localStorage() async {
try {
await SharedPreferences.getInstance().then((SharedPreferences prefs) {
preferences = prefs;
_user = preferences.getString(USER).toString();
});
setState(() {
user = jsonDecode(_user);
});
} catch (e) {
log('data: $e');
}
}
#override
void initState() {
// TODO: implement initState
super.initState();
_localStorage();
}
What am I missing?