我在Angular上写了项目，我尝试显示文章，但有这样的错误，也无法创建文章。浏览器控制台中显示以下错误：
TypeError: Cannot read property 'tasks' of null
at task.ts:30
at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invoke (zone.js:388)
at Object.onInvoke (core.js:3654)
at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invoke (zone.js:387)
at Zone.push../node_modules/zone.js/dist/zone.js.Zone.run (zone.js:138)
at zone.js:872
at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:421)
at Object.onInvokeTask (core.js:3645)
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)
ERROR Error: Uncaught (in promise): Cannot read property 'tasks' of null
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:3645)
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)
at ZoneTask.push../node_modules/zone.js/dist/zone.js.ZoneTask.invokeTask [as invoke] (zone.js:500)
at invokeTask (zone.js:1540)
最有可能导致错误的代码：
26 loadTasks(): Promise<Task[]> {
27 const url = `${this.tasksUrl}?access_token=${localStorage.getItem('token')}`;
28 return this.http.get(url)
29 .toPromise()
30 .then(res => res.json().tasks as Task[])
31 .catch(error => this.handleError(error, 'Could not load tasks!'));
32 }
33 getTask(id: number): Promise<Task> {
34 const url = `${this.tasksUrl}/${id}?access_token=${localStorage.getItem('token')}`;
35 return this.http.get(url)
36 .toPromise()
37 .then(res => res.json() as Task)
38 .catch(error => this.handleError(error, 'Could not load task!'));
39 }
40 create(task): Promise<Task | void> {
41 task['due_date'] = task['due_date']['formatted'];
42 let body = JSON.stringify({task: task});
43 const url = `${this.tasksUrl}?access_token=${localStorage.getItem('token')}`;
44 return this.http.post(url, body, { headers: this.headers })
45 .toPromise()
46 .then(res => res.json() as Task)
47 .catch(error => {
48 this.handleError(error, 'Could not create task!')
49 });
50 }