PopupMenuButton -> show dialog on popupMenuItem tap

example.dart

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  void showDialog(BuildContext context) {
    WidgetsBinding?.instance?.addPostFrameCallback((_) {
      showCupertinoDialog(
          context: context,
          builder: (context) {
            return CupertinoAlertDialog(
              title: const Icon(CupertinoIcons.info_circle),
              content: const Text(
                'Hello User, Welcome',
                textAlign: TextAlign.center,
              ),
              actions: [
                CupertinoDialogAction(
                  isDefaultAction: true,
                  onPressed: () => Navigator.pop(context),
                  child: const Text('Thanks'),
                ),
              ],
            );
          });
    });
  }

  @override
  Widget build(BuildContext context) {
    return PopupMenuButton(
      itemBuilder: (context) {
        return [
          PopupMenuItem(
              child: const Text('Item 0'),
              onTap: () {
                WidgetsBinding?.instance?.addPostFrameCallback((_) {
                  showCupertinoDialog(
                      context: context,
                      builder: (context) {
                        return CupertinoAlertDialog(
                          title: const Icon(CupertinoIcons.info_circle),
                          content: const Text(
                            'Hello User, Welcome',
                            textAlign: TextAlign.center,
                          ),
                          actions: [
                            CupertinoDialogAction(
                              isDefaultAction: true,
                              onPressed: () => Navigator.pop(context),
                              child: const Text('Thanks'),
                            ),
                          ],
                        );
                      });
                });
              }),
          PopupMenuItem(child: const Text('Item 1')),
          PopupMenuItem(child: const Text('Item 2')),
          PopupMenuItem(child: const Text('Item 3')),
        ];
      },
    );
  }
}