show_dialog_on_app_start.dart

import 'package:flutter/material.dart';

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

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

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  final navKey = GlobalKey<NavigatorState>();
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance!.addPostFrameCallback((timeStamp) {
      showDialog(
        context: navKey.currentContext!,
        builder: (context) => const AlertDialog(
          title: Text('AlertDialog'),
          content: Text('This is an alert dialog.'),
        ),
      );
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      navigatorKey: navKey,
      title: 'Material App',
      home: Scaffold(
        appBar: AppBar(title: const Text('Material App Bar')),
        body: const Center(child: Text('Hello World')),
      ),
    );
  }
}