无法在Scaffold小部件后面堆叠图像

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Stack Example'),
        ),
        body: Stack(
          children: [
            Positioned.fill(
              child: Image.network(
                'https://example.com/image.jpg',
                fit: BoxFit.cover,
              ),
            ),
            Center(
              child: Text(
                'Hello, World!',
                style: TextStyle(fontSize: 24),
              ),
            ),
          ],
        ),
      ),
    );
  }
}