在flutter中，如何在卡片内的AppBar和TextFormField之间留出空格？

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('Card Example'),
        ),
        body: Center(
          child: Card(
            child: Padding(
              padding: EdgeInsets.all(16.0), // 调整空白区域的大小
              child: Column(
                children: [
                  TextFormField(
                    decoration: InputDecoration(
                      labelText: 'Username',
                    ),
                  ),
                  SizedBox(height: 16.0), // 在AppBar和TextFormField之间添加空白
                  TextFormField(
                    decoration: InputDecoration(
                      labelText: 'Password',
                    ),
                  ),
                ],
              ),
            ),
          ),
        ),
      ),
    );
  }
}