Невозможно прочитать значения из firebase в списке Angular 6 неопределенного типа ошибка в обещании

Question

Я получаю приведенную ниже ошибку при чтении данных из firebase.
Ошибка: Uncaught (в обещании): TypeError: Невозможно прочитать свойство 'список' из неопределенного

Я создаю класс репозитория со следующейcode.

import { AngularFireDatabase } from 'angularfire2/database';

export class TopListRepository {

    topList$;
    constructor(private db: AngularFireDatabase) {
        this.topList$ =  db.list('/TopList');
    }

    get(listName) {
        console.log('Inside Repo class TopListRepository');
        console.log('Parameter val: ' + listName);
        console.log('DB object: ' + this.db);
        return this.db.object('/TopList/' + listName).valueChanges();
      }
}

Пока ниже указан мой класс BO,

import { Observable } from 'rxjs';
import { TopListRepository } from '../respository/topListRepository';
import { AngularFireDatabase } from 'angularfire2/database';

export class TopListBO {

    topListRepo: TopListRepository;
    private db: AngularFireDatabase;

    constructor() {}

    getTopList(listName) {
        const repo = new TopListRepository(this.db);
        return repo.get(listName);
    }
 }
}

Я вызываю этот класс BO из моего метода get класса обслуживания,

get(listName) {
    const boClass = new TopListBO();
    return boClass.getTopList(listName);

  }

ниже подробно об ошибке,

ERROR Error: Uncaught (in promise): TypeError: Cannot read property 'list' of undefined
TypeError: Cannot read property 'list' of undefined
    at new TopListRepository (topListRepository.ts:7)
    at TopListBO.push../src/app/models/TopListBO.ts.TopListBO.getTopList (TopListBO.ts:13)
    at ToplistsService.push../src/app/services/toplists.service.ts.ToplistsService.get (toplists.service.ts:19)
    at new TopplayersComponent (topplayers.component.ts:20)
    at createClass (core.js:10148)
    at createDirectiveInstance (core.js:10033)
    at createViewNodes (core.js:11255)
    at callViewAction (core.js:11571)
    at execComponentViewsAction (core.js:11490)
    at createViewNodes (core.js:11283)
    at new TopListRepository (topListRepository.ts:7)
    at TopListBO.push../src/app/models/TopListBO.ts.TopListBO.getTopList (TopListBO.ts:13)
    at ToplistsService.push../src/app/services/toplists.service.ts.ToplistsService.get (toplists.service.ts:19)
    at new TopplayersComponent (topplayers.component.ts:20)
    at createClass (core.js:10148)
    at createDirectiveInstance (core.js:10033)
    at createViewNodes (core.js:11255)
    at callViewAction (core.js:11571)
    at execComponentViewsAction (core.js:11490)
    at createViewNodes (core.js:11283)
    at resolvePromise (zone.js:814)
    at resolvePromise (zone.js:771)
    at zone.js:873
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:421)
    at Object.onInvokeTask (core.js:4053)
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:420)
    at Zone.push../node_modules/zone.js/dist/zone.js.Zone.runTask (zone.js:188)
    at drainMicroTaskQueue (zone.js:595)
defaultErrorLogger @ core.js:1598