课程5-16，点击右下角按钮的时候报错

代码如下：

import 'dart:io';

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

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

class PhotoPage extends StatefulWidget {
  @override
  _PhotoPageState createState() => _PhotoPageState();
}

class _PhotoPageState extends State<PhotoPage> {
  File _image;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Photo App',
      theme:
          ThemeData(primarySwatch: Colors.blue, brightness: Brightness.light),
      home: Scaffold(
        appBar: AppBar(
          title: Text('Image Picker Example'),
        ),
        body: Center(
          child:
              _image == null ? Text('No image selected.') : Image.file(_image),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: _pickImage,
          tooltip: '选择图片',
          child: Icon(Icons.add_a_photo),
        ),
      ),
    );
  }

  _pickImage() {
    showModalBottomSheet(
        context: context,
        builder: (context) => Container(
              height: 160,
              child: Column(
                children: <Widget>[
                  _listItem('拍照', true),
                  _listItem('从相册选择', false)
                ],
              ),
            ));
  }

  _listItem(String title, bool camera) {
    return GestureDetector(
      child: ListTile(
        leading: Icon(camera ? Icons.camera_alt : Icons.photo_library),
        title: Text(
          title,
          textDirection: TextDirection.ltr,
        ),
        onTap: () => getImage(camera),
      ),
    );
  }

  Future getImage(bool camera) async {
    var image = await ImagePicker.pickImage(
        source: camera ? ImageSource.camera : ImageSource.gallery);

    setState(() {
      _image = image;
    });
  }
}

另外，如果build方法最外层返回的Scaffold不使用MaterialApp包裹的话，也会报类似的错误