Navigator bug

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Navigators")),
      body: Column(
        mainAxisSize: MainAxisSize.max,
        children: [
          Expanded(child: TopPanel()),
          Expanded(child: BottomPanel()),
        ],
      ),
    );
  }
}

class TopPanel extends StatefulWidget {
  @override
  _TopPanelState createState() => _TopPanelState();
}

class _TopPanelState extends State<TopPanel> {
  final GlobalKey<NavigatorState> _navigatorKey = GlobalKey<NavigatorState>();
  TextEditingController controller;
  @override
  void initState() {
    super.initState();
    controller = TextEditingController();
  }

  @override
  void dispose() {
    controller?.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Navigator(
        key: _navigatorKey,
        initialRoute: "/",
        onGenerateRoute: (settings) {
          return MaterialPageRoute(builder: (context) {
            return Container(
              color: Colors.green,
              child: Center(
                child: TextFormField(
                  controller: controller,
                  decoration: InputDecoration(hintText: "Input"),
                ),
              ),
            );
          });
        });
  }
}

class BottomPanel extends StatefulWidget {
  BottomPanel({Key key}) : super(key: key);

  _BottomPanelState createState() => _BottomPanelState();
}

class _BottomPanelState extends State<BottomPanel> {
  TextEditingController controller;

  @override
  void initState() {
    super.initState();
    controller = TextEditingController();
  }

  @override
  void dispose() {
    controller?.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.red,
      child: Center(
        child: TextFormField(
          controller: controller,
          decoration: InputDecoration(hintText: "2nd input"),
        ),
      ),
    );
  }
}