Ionic2 AlertController 2未捕获类型错误：无法读取属性'；创建'；未定义的

我试图在退出应用程序时进行验证，但在使用AlertController ionic 2:时遇到错误，我按照

对不起，我英语不好

我的错误：

Uncaught TypeError: Cannot read property 'create' of undefined
at Object.MyApp.exit [as fn] (main.js:143575)
at Platform.runBackButtonAction (main.js:2347)
at main.js:2022
at SafeSubscriber.schedulerFn [as _next] (main.js:54953)
at SafeSubscriber.__tryOrUnsub (main.js:1388)
at SafeSubscriber.next (main.js:1337)
at Subscriber._next (main.js:1290)
at Subscriber.next (main.js:1254)
at EventEmitter.Subject.next (main.js:7763)
at EventEmitter.emit (main.js:54927)
at main.js:104021
at t.invoke (polyfills.js:3)
at Object.onInvoke (main.js:38895)
at t.invoke (polyfills.js:3)
at e.run (polyfills.js:3)

我的代码app.component.ts：

试试这段代码

 exit(){
   this.alertCtrl.create({
     title :'' , 
     subTitle:'',
     buttons:['']
   }).then(alertEt =>{
    alertEt.present();
   }) 
}

试试这个