'package:flutter/src/widgets/will_pop_scope.dart': Failed assertion: line 59 pos 12: '_route == ModalRoute.of(context)': is not true.

form_pop_example.dart

import 'package:flutter/material.dart';

void main() => runApp(FormPopExample());

class FormPopExample extends StatefulWidget {
  @override
  _FormPopExampleState createState() => _FormPopExampleState();
}

class _FormPopExampleState extends State<FormPopExample> {
  GlobalKey<NavigatorState> navState;

  @override
  void initState() {
    super.initState();

    navState = GlobalKey<NavigatorState>();
  }

  Widget pageContent(
    BuildContext context,
    Widget content,
  ) =>
      Navigator(
        key: navState,
        onGenerateRoute: (route) {
          return;
        },
      );

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(leading: BackButton(), title: Text('form pop')),
        body: Builder(
          builder: (context) => RaisedButton(
            onPressed: () => Navigator.of(context).push(
              MaterialPageRoute<Object>(
                maintainState: true,
                builder: (context) => FormPop(),
              ),
            ),
            child: Hero(
              tag: 'form',
              child: Text('push form'),
            ),
          ),
        ),
      ),
    );
  }
}

class FormPop extends StatefulWidget {
  @override
  _FormPopState createState() => _FormPopState();
}

class _FormPopState extends State<FormPop> {
  final _formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(leading: BackButton(), title: Text('form pop')),
      body: Hero(
        tag: 'form',
        child: Form(
          key: _formKey,
          child: Container(
            child: Text("wooweee look at me"),
          ),
        ),
      ),
    );
  }
}

minimal reproduction of flutter/flutter#49952