Flutter 异常: The getter ‘modalBarrierDismissLabel’ was called on null. Receiver: null Tried calling:

一般出现这种问题都是直接在 MaterialApp 层显示 Dialog 造成的，因为显示 Dialog 传入的是 MaterialApp 层的 context，所以会出现这种错误。

1.问题场景

import 'package:flutter/cupertino.dart';
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('Demo'),
        ),
        body: Center(
          child: RaisedButton(
            onPressed: () {
              showCupertinoDialog(
                  ///此处传入的是 MaterialApp 的 context
                  context: context,
                  builder: (context) {
                    /// 此处省略弹窗代码
                  }
              }
            },
            child: Text('点击显示弹窗'),
          ),
        ),
      ),
    );
  }
}

因为 context 是 MaterialApp 的，所以会报这样的错：

════════ Exception caught by gesture ════════
The following NoSuchMethodError was thrown while handling a gesture:
The getter 'modalBarrierDismissLabel' was called on null.
Receiver: null 
Tried calling: modalBarrierDismissLabel

2.解决方案

直接在 MaterialApp 的 home 中在传入一个继承于 StatelessWidget 或者是 StatefulWidget 的部件即可。如：

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: DialogPage(),
    );
  }
}

class DialogPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Demo'),
      ),
      body: Center(
        child: RaisedButton(
          onPressed: () {
            _showDialog(context);
          },
          child: Text('点击显示弹窗'),
        ),
      ),
    );
  }
}

void _showDialog(widgetContext) {
  /// 显示 IOS 风格的 CupertinoAlertDialog
  showCupertinoDialog(
    context: widgetContext,
    builder: (context) {
      return CupertinoAlertDialog(
        title: Text('确认删除'),
        actions: [
          CupertinoDialogAction(
            child: Text('确认'),
            onPressed: () {
              Navigator.of(context).pop();
            },
          ),
          CupertinoDialogAction(
            child: Text('取消'),
            isDestructiveAction: true,
            onPressed: () {
              Navigator.of(context).pop();
            },
          ),
        ],
      );
    },
  );
}