如何修复在Navcontroller ionic中找不到的推送

Question

使用此代码时出现以下错误。我还尝试将push函数更改为setRoot，并且仍然存在相同的错误。

如果用户未登录但无法登录，我想导航到主页即登录页面。

控制台显示它输入了if语句，但navctrl命令没有得到执行。

MyApp_Host.ngfactory.js? [sm]:1 ERROR TypeError: Cannot read property 'push' of undefined

这是我的代码：

import { Component, ViewChild } from '@angular/core';
import { IonicPage, NavController, NavParams, AlertController} from 'ionic-angular';
import * as firebase from 'firebase/app';
import { HomePage } from '../home/home';

/**
 * Generated class for the MainPage page.
 *
 * See https://ionicframework.com/docs/components/#navigation for more info on
 * Ionic pages and navigation.
 */

@IonicPage()
@Component({
  selector: 'page-main',
  templateUrl: 'main.html',
})

export class MainPage {



  constructor(public navParams: NavParams, public alertCtrl: AlertController, public navController:NavController) {

  }

  ionViewDidLoad() {

    firebase.auth().onAuthStateChanged(function(user) {
      if (user) {
        console.log(user);
        console.log('entered in if');
        // User is signed in.
      } else {
        console.log(user);
        console.log('entered in else');
        this.navController.push(HomePage);

        // No user is signed in.
      }
    });

    console.log('ionViewDidLoad MainPage')


  }

}