How to fix push not found in Navcontroller ionic - firebase

Question

I am getting the following error while using this code. I have also tried to change the push function to setRoot and the same error still persists.
I want to navigate to the home page which is the login page in case the user is not signed in but unable to do so.
The console shows that it entered the if statement but the navctrl command is not getting executed.
MyApp_Host.ngfactory.js? [sm]:1 ERROR TypeError: Cannot read property 'push' of undefined
This my code:
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')
}
}