A self-contained runnable Flutter example of using StatefulBuilder to make a wrapper context for Form.of (line 21) to return the parent form (line 11)

main.dart

import "package:flutter/material.dart";
//https://github.com/flutter/flutter/issues/26772#issuecomment-455674193
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: StatefulBuilder(
          builder: (context,setState){return
            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);
                  },),
              ]
            );
          },
        )  
      ),
    );
  }
}