Form.of Returning NULL Error Demonstration

main.dart

import "package:flutter/material.dart";
main() {runApp(MaterialApp(home:HomeScreen()));}
class HomeScreen extends StatefulWidget {@override createState() {return _HomeScreen();}}
class _HomeScreen extends State<StatefulWidget> {
  String data;
  final GlobalKey<FormState> formKey =  GlobalKey<FormState>();
  @override Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Home")),
      body:Form(key:this.formKey,
        child:ListView(
          children: <Widget>[
              TextFormField(onSaved: (v){this.data = v;}),
              RaisedButton(child: Text('Logout'),
                onPressed: () {
                  //formKey.currentState.save();
                  Form.of(context).save();
                  print("********** Data=" + this.data);
              },),
          ]
        )  
      ),
    );
  }
}