IONIC 4: Cannot read Property 'get' of Undefined

Question

I’m trying to get some database information through the function get(), but when I try to run it, gets the error:

ERROR TypeError: Cannot read property 'get' of undefined
  at HomeTabPage.push../src/app/home-tab/home-tab.page.ts.HomeTabPage.ngOnInit (home-tab.page.ts:18)

^{Full error print on console}

In the * ngFor I tried using the Elvis parameter, but it didn’t work. I already passed my main function of ngOnInit and declared another function to reference, but it did not work.

Typescript/HTML

export class HomeTabPage implements OnInit {
  public advertList: Array<any>;

  constructor(private navCtrl: NavController, public adService: AdvertisingService) { 

  }

  ngOnInit(){
    this.adService.getAdAllList().get().then(advertListSnapshot => {
      this.advertList = [];
      advertListSnapshot.forEach(snap =>{
        this.advertList.push({
          id: snap.id,
          name: snap.data().name,
          price: snap.data().price,
          description: snap.data().description,
          image: snap.data().picture
        });
      });
    });
  }
}

      <ion-list>
        <ion-item *ngFor="let advert of advertList">
          <ion-thumbnail slot="start">
            <ion-img src="{{advert?.image}}"></ion-img>
          </ion-thumbnail>
          <ion-label>
            <h2 >{{advert?.name}}</h2>
            <h3 >{{advert?.price}}</h3>
            <p >{{advert?.description}}</p>
          </ion-label>
        </ion-item>
      </ion-list>

service ts.

export class AdvertisingService {
  public adAllListRef: firebase.firestore.CollectionReference;
  constructor() {
    firebase.auth().onAuthStateChanged(user => {
      if (user) {
        this.adAllListRef = firebase.firestore().collection(`/adverts/adAll/adList`);
      }
    });
  }

  //cRud -> Read
  getAdAllList(): firebase.firestore.CollectionReference {
    return this.adAllListRef;
  }
}