Hero inserts after dispose

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(routes: {
    '/': (context) => Scaffold(
            body: Center(
          child: FlatButton(
            child: const Text('open'),
            onPressed: () => Navigator.of(context).pushNamed('/page'),
          ),
        )),
    '/page': (context) => Page()
  }, initialRoute: '/'));
}

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

  @override
  _PageState createState() => _PageState();
}

class _PageState extends State<Page> {
  ValueNotifier notifier;
//  FixedValueNotifier notifier;

  @override
  void initState() {
    notifier = ValueNotifier(null);
//    notifier = FixedValueNotifier(null);
    super.initState();
  }

  @override
  void dispose() {
    notifier.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) => Scaffold(
          body: Column(mainAxisAlignment: MainAxisAlignment.spaceEvenly, children: [
        FlatButton(
          child: const Text('push'),
          onPressed: () => Navigator.of(context).pushNamed('/page'),
        ),
        FlatButton(
          child: const Text('popUntil'),
          onPressed: () => Navigator.of(context).popUntil(ModalRoute.withName('/')),
        ),
        Hero(
            tag: 'tag',
            child: AnimatedBuilder(
              animation: notifier,
              builder: (context, child) => Container(),
            ))
      ]));
}

class FixedValueNotifier<T> extends ValueNotifier<T> {
  bool disposed;

  FixedValueNotifier(value)
      : disposed = false,
        super(value);

  @override
  void dispose() {
    disposed = true;
    super.dispose();
  }

  @override
  void removeListener(listener) {
    if (disposed) return;
    super.removeListener(listener);
  }
}