Flutter| Dio : Invalid argument(s) (value): Must not be null

i am using shared preference to pass data from one screen to other, it was working perfectly few days ago, but now it is sending this error

Invalid argument(s) (value): Must not be null

i'm calling login api, it is running sucessfully, i am getting api data on its response but it goes to catch part after print the data, it is not navigating to other screen, here is the login function code

SharedPreferences myPrefs;
Future login() async {
    Dio dio = new Dio();
    
    try {  
      data = {
        'username':"Munib khatri",
        'password':"Munib123",
        'date': formattedDate
      }; 
     await dio
          .post(localhostUrlLogin, data: json.encode(data),)
          .then((onResponse) async {
            print(onResponse.data);

        String name = (onResponse.data['User']['username']);        
        String email = (onResponse.data['User']['email']);
        String designation = (onResponse.data['User']['Designation']);
        String token = (onResponse.data['AccessToken']);

           //here i am calling another api
         data={ 
          "token":token,
          "username":name,
          "leave":"Approved",
          "type":"maternity"
        };
         dio
          .post(localHostUrlleaveCount, data: json.encode(data))
            .then((onResponse) async {
              int sickCount=(onResponse.data['sick']);
              int annualCount=(onResponse.data['annual']);             
                       
              await myPrefs.setInt('sickCount', sickCount);
              await myPrefs.setInt('annualCount', annualCount);
            

            }).catchError((onerror){
          });

        await myPrefs.setString('name', name);
        await myPrefs.setString('email', email);
        await myPrefs.setString('designation', designation);
      

        Navigator.push(
              context, new MaterialPageRoute(builder: (context) => Navigation()));
      
      });
    } catch (e) {
          print(e.toString()); 
      
    }
  }

Navigation:

class Navigation extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: new ThemeData(primaryColor: Colors.blue),
      home: EmployeeNavigation(),
    );
  }
}

int _selectedTab = 0;
final _pageOptions = [Home(), location(), Profile()];

String getname = "";
String getemail = "";
String getdesignation = "";
String getaccesstoken = "";

// ignore: must_be_immutable
class EmployeeNavigation extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return EmployeeNavigationState();
  }
}

class EmployeeNavigationState extends State<EmployeeNavigation> {
  var email;
  var designation;
  var date;
  bool valuefirst = false;
  String backtext = "";

  @override
  Widget build(BuildContext context) {
    return WillPopScope(
        onWillPop: () async {
          if (_selectedTab == 0) {
            return true;
          }
          setState(() {
            _selectedTab = 0;
          });
          return false;
        },
        child: Scaffold(
            drawer: NavigationDrawerWidget(), //this is a drawer file
            body: _pageOptions[_selectedTab],
            bottomNavigationBar: BottomNavigationBar(
              backgroundColor: Colors.blue[50],
              type: BottomNavigationBarType.fixed,
              currentIndex: _selectedTab,
              onTap: (value) {
                print(value);
                setState(() {
                  _selectedTab = value;
                });
              },
              items: [
                BottomNavigationBarItem(icon: Icon(Icons.home), label: "Home"),
                BottomNavigationBarItem(
                    icon: Icon(Icons.location_on), label: "Location"),
                BottomNavigationBarItem(
                  icon: Icon(Icons.person),
                  label: "Profile",
                ),
              ],
            )));
  }
}

i am getting response of both api, but can't navigate to other screen.