Attempt to close snackbar when closing view

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(App());
}

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(home: const HomePage(), routes: {
      '/detail': (_) => const DetailPage(),
    });
  }
}

class HomePage extends StatelessWidget {
  const HomePage({
    Key? key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Home')),
      body: SizedBox.expand(
        child: TextButton(
          child: const Text('See detail'),
          onPressed: () {
            Navigator.pushNamed(context, '/detail');
          },
        ),
      ),
    );
  }
}

class DetailPage extends StatefulWidget {
  const DetailPage({
    Key? key,
  }) : super(key: key);

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

class _DetailPageState extends State<DetailPage> {
  final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance!.addPostFrameCallback((_) {
      ScaffoldMessenger.of(_scaffoldKey.currentContext!).showSnackBar(SnackBar(
        content: const Text('Entered detail page'),
        duration: const Duration(days: 1),
        action: SnackBarAction(
            label: 'Close',
            onPressed: () {
              ScaffoldMessenger.of(_scaffoldKey.currentContext!)
                  .hideCurrentSnackBar();
            }),
      ));
    });
  }

  @override
  void dispose() {
    ScaffoldMessenger.of(_scaffoldKey.currentContext!).hideCurrentSnackBar();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      key: _scaffoldKey,
      appBar: AppBar(title: const Text('Detail')),
      body: const SizedBox.expand(
        child: Center(child: Text('Detail page')),
      ),
    );
  }
}