flutter dialog and option menu dispose issue

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return OrientationBuilder(builder: (context, orientation) {
      bool isLandscape = orientation == Orientation.landscape;
      return isLandscape ? Landscape() : Portrait();
    });
  }
}

class Portrait extends StatefulWidget {
  @override
  _PortraitState createState() => _PortraitState();
}

class _PortraitState extends State<Portrait> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: buildTitle(),
        actions: <Widget>[_buildOptionMenu(context)],
      ),
      body: GestureDetector(
        onLongPress: () {
          showDialog(
            context: context,
            builder: (context) => AlertDialog(
              content: buildTitle(),
            ),
          );
        },
        child: Container(
          color: Colors.blue.withOpacity(0.4),
          child: Stack(
            fit: StackFit.expand,
            children: <Widget>[
              Center(
                child: buildTitle(),
              ),
            ],
          ),
        ),
      ),
    );
  }

  Widget _buildOptionMenu(BuildContext context) {
    return PopupMenuButton(itemBuilder: (context) {
      var list = <String>['Portrait-Item-1', 'Portrait-Item-2'];
      return list
          .map<PopupMenuEntry<String>>(
            (e) => PopupMenuItem<String>(
              child: Text(e),
            ),
          )
          .toList();
    });
  }

  Text buildTitle() => Text('Portrait');
}

class Landscape extends StatefulWidget {
  @override
  _LandscapeState createState() => _LandscapeState();
}

class _LandscapeState extends State<Landscape> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: buildTitle(),
        actions: <Widget>[_buildOptionMenu(context)],
      ),
      body: GestureDetector(
        onLongPress: () {
          showDialog(
            context: context,
            builder: (context) => AlertDialog(
              content: buildTitle(),
            ),
          );
        },
        child: Container(
          color: Colors.orange.withOpacity(0.3),
          child: Stack(
            fit: StackFit.expand,
            children: <Widget>[
              Center(
                child: buildTitle(),
              ),
            ],
          ),
        ),
      ),
    );
  }

  Text buildTitle() => Text('Landscape');

  Widget _buildOptionMenu(BuildContext context) {
    return PopupMenuButton(itemBuilder: (context) {
      var list = <String>[
        'Landscape-Item-1',
        'Landscape-Item-2',
        'Landscape-Item-3',
      ];
      return list
          .map<PopupMenuEntry<String>>(
            (e) => PopupMenuItem<String>(
              child: Text(e),
            ),
          )
          .toList();
    });
  }
}