錯誤類型錯誤：無法讀取 null 的屬性“推送”

Question

我正在嘗試推送到列表，但出現以下錯誤：

import { Injectable } from '@angular/core';
// import { AngularFire, FirebaseListObservable, FirebaseObjectObservable, AngularFireDatabase } from "angularfire2";
import { AngularFireDatabase, FirebaseListObservable, FirebaseObjectObservable } from 'angularfire2/database';
import { Project } from './project'

@Injectable()
export class ProjectService {

  private basePath: string = '/projects';

  //items: FirebaseListObservable<Project[]> = null; //  list of objects
  items: FirebaseListObservable<Project[]> = null; //  list of objects
  item: FirebaseObjectObservable<Project> = null; //   single object

  constructor(private db: AngularFireDatabase) {}

  // Return an observable list with optional query
  // You will usually call this from OnInit in a component
  getItemsList(query={}): FirebaseListObservable<Project[]> {
    this.items = this.db.list('/projects', {
      query: query
    });
    return this.items
  }

  // Return a single observable item
  getItem(key: string): FirebaseObjectObservable<Project> {
    const itemPath =  `${this.basePath}/${key}`;
    this.item = this.db.object(itemPath)
    return this.item
  }

  // Create a bramd new item
  createItem(item: Project): void  {
    this.items.push(item)
      .catch(error => this.handleError(error))
  }


  // Update an exisiting item
  updateItem(key: string, value: any): void {
    this.items.update(key, value)
      .catch(error => this.handleError(error))
  }

  // Deletes a single item
  deleteItem(key: string): void {
      this.items.remove(key)
        .catch(error => this.handleError(error))
  }

  // Deletes the entire list of items
  deleteAll(): void {
      this.items.remove()
        .catch(error => this.handleError(error))
  }


  // Default error handling for all actions
  private handleError(error) {
    console.log(error)
  }


}

錯誤：

錯誤類型錯誤：無法讀取 null 的屬性“推送”
在
ProjectService.webpackJsonp.../../../../../src/app/projects/shared/project.service.ts.ProjectService.createItem (project.service.ts:36)
在
ProjectFormComponent.webpackJsonp.../../../../../src/app/projects/project-form/project-form.component.ts.ProjectFormComponent.createItem (project-form.component.ts:20 )
在 Object.eval [作為 handleEvent] (ProjectFormComponent.html:8)
在 handleEvent (core.es5.js:12047)
在 callWithDebugContext (core.es5.js:13508)
在 Object.debugHandleEvent [as handleEvent] (core.es5.js:13096)
在 dispatchEvent (core.es5.js:8659)
在 core.es5.js:9270
在 HTMLButtonElement。 (平台瀏覽器.es5.js:2668)
在
ZoneDelegate.webpackJsonp.../../../../zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:424)

我已經嘗試獲取空聲明，它會生成另一個未定義的錯誤。

Answer 1

我找到了一個解決方案：

我在構造函數中啟動了列表：

constructor(private db: AngularFireDatabase) {
        this.items = db.list('/projects');
   }