To avoid shows console error if the user is not authenticated to get listings from Firebase. [Angular 2 & Firebase App [Part 4] - Firebase Authentication]

listings.component.ts

import { Component, OnInit } from '@angular/core';
import { FirebaseService } from '../../services/firebase.service';

@Component({
  selector: 'app-listings',
  templateUrl: './listings.component.html',
  styleUrls: ['./listings.component.css']
})
export class ListingsComponent implements OnInit {

  listings:any;

  constructor(private firebaseService: FirebaseService) { }

  ngOnInit() {
    this.firebaseService.getListings().subscribe(listings => {
      console.log(listings);
      this.listings = listings;
    }, err => { 
      console.log(err);
    });
  }

}