Flutter w/ MobX:出现异常时，Snackbar不显示

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

// 创建一个全局的异常处理器
final globalErrorHandler = (Object error, StackTrace stackTrace) {
  // 在这里显示Snackbar
  ScaffoldMessenger.of(context).showSnackBar(
    SnackBar(
      content: Text('出现异常：$error'),
    ),
  );
};

void main() {
  // 初始化MobX
  WidgetsFlutterBinding.ensureInitialized();
  // 设置全局异常处理器
  FlutterError.onError = globalErrorHandler;

  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter w/ MobX',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  final counter = Observable(0);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter w/ MobX'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              '点击按钮增加计数器的值：',
            ),
            Observer(
              builder: (_) => Text(
                '${counter.value}',
                style: Theme.of(context).textTheme.headline4,
              ),
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          // 在这里故意引发一个异常
          throw Exception('这是一个故意引发的异常');
        },
        child: Icon(Icons.add),
      ),
    );
  }
}