为什么Firebase没有将数据添加到数据库？ Firebase服务代码有什么问题？

Question

我正在开发一个Angular + Firebase项目。我在firebase数据库上传数据时遇到错误。这是我在Angular2中的“firebase”服务的代码。

.FirstOrDefault()

当我点击HTML模板中的“提交”按钮时，会调用“onAddSubmit”函数，该函数会进一步调用FirebaseService的“addListing”方法。

import { AngularFirestore } from 'angularfire2/firestore';
import { Injectable } from '@angular/core';
import { Observable } from 'rxjs/Observable';
import { AngularFireDatabaseModule, AngularFireDatabase } from 'angularfire2/database';
import { AngularFireObject, AngularFireList } from 'angularfire2/database';
import * as firebase from 'firebase';

@Injectable()
export class FirebaseService {
  listings ;
  listing: Observable<any[]>;
  folder: any;

  constructor(private db: AngularFireDatabase) {
    this.folder = 'listingimages';
   }

  getListings(){
      //this.listings = this.db.list('/listings').valueChanges() as Observable<Listing[]>
      this.listings = this.db.list('/listings').snapshotChanges().map(actions => {
        return actions.map(action => ({ key: action.key, ...action.payload.val() }));
      })
      return this.listings;
  }

  getListingDetails(id){
    this.listing = this.db.object('/listings/'+id).valueChanges() as Observable<Listing[]>;
    return this.listing;

  }

  addListing(listing){
    //Create Root Reference     
        let storageRef = firebase.storage().ref();
    for(let selectedFile of [(<HTMLInputElement>document.getElementById('image')).files[0]]){
      let path = `/${this.folder}/${selectedFile.name}`;
      let iRef = storageRef.child(path);
      iRef.put(selectedFile).then((snapshot) => {
        listing.image = selectedFile.name;
        listing.path = path;
        return this.listings.push(listing);

        });
    }
  }

}

interface Listing{
    $key?: string;
    title?: string;
    type?: string;
    image?: string;
    city?: string;
    owner?: string;
    bedrooms?: string
}

我没有得到函数定义的错误，但是没有任何东西在firebase数据库中添加。提交时显示的错误如下：

感谢。