accessing a private method from another class flutter - class

Question

I'm new to flutter.I have here 3 classes which are the Login(), MainMenu() which is the screen after already logged, and this MyDrawer()which is a drawer of my App.
What I'm trying to do is I want to access the signOut() method from Login(). How would I do it or what should I do to redesign my code. I've tried below accessing it and it receives and exception The method 'call' was called on null.
This is a code snippet from my full code:
class Login extends StatefulWidget {
#override
_LoginState createState() => _LoginState();
}
enum LoginStatus { notSignIn, signIn }
class _LoginState extends State<Login> {
LoginStatus _loginStatus = LoginStatus.notSignIn;
String email, password;
final _key = new GlobalKey<FormState>();
bool _secureText = true;
signOut() async {
SharedPreferences preferences = await SharedPreferences.getInstance();
setState(() {
_loginStatus = LoginStatus.notSignIn;
});
}
#override
void initState() {
super.initState();
}
#override
Widget build(BuildContext context) {
switch (_loginStatus) {
case LoginStatus.notSignIn:
return Scaffold(
backgroundColor: Colors.cyan,
body: Center(
child: ListView(
shrinkWrap: true,
padding: EdgeInsets.all(15.0),
children: <Widget>[
Center(
child: Container(
padding: const EdgeInsets.all(8.0),
color: Colors.cyan,
child: Form(
key: _key,
break;
case LoginStatus.signIn:
return MainMenu();
break;
}
}
}
class MainMenu extends StatefulWidget {
#override
_MainMenuState createState() => _MainMenuState();
}
class _MainMenuState extends State<MainMenu> {
#override
Widget build(BuildContext context) {
return MaterialApp(
debugShowCheckedModeBanner: false,
home: Scaffold(
)
)
}
class MyDrawer extends StatefulWidget {
final Function onTap;
final VoidCallback signOut;
MyDrawer(
{this.onTap,this.signOut
});
#override
_MyDrawerState createState() => _MyDrawerState();
}
class _MyDrawerState extends State<MyDrawer> {
signOut() {
setState(() {
widget.signOut();
});
}
#override
void initState() {
super.initState();
}
#override
Widget build(BuildContext context) {
return SizedBox(
width: MediaQuery
.of(context)
.size
.width * 0.7,
child: Drawer(
child: Container(
height:100,
color: Colors.white,
child: ListView(
padding: EdgeInsets.all(0),
children: <Widget>[
ListTile(
leading: Icon(Icons.exit_to_app,color: Colors.cyan, size: 30.0),
onTap: () {
signOut();
},
title: Text("Logout",
style: TextStyle(color: Colors.black,fontWeight: FontWeight.w500, fontSize: 18),
),
),
],
),
),
),
);
}
}
I'm really stuck with this problem. Any help would be greatly appreciated. Thanks!