Angular 6 – ERROR TypeError：无法读取undefined的属性“value”

我是Angular和Typescript的新手,无法到达此控制台错误的底部：“ERROR TypeError：无法读取未定义的属性’值’

我打电话给一个愚蠢的服务,回到查克诺里斯的笑话.这实际上工作正常.但是我收到的是Typescript控制台错误.

我在这里转载：
https://stackblitz.com/edit/angular-chuck

非常感谢您的期待.

data.service.ts

import { Injectable } from '@angular/core';
import { httpClient } from '@angular/common/http';

import { DataModel } from './data.model';

@Injectable()
export class Dataservice {
  constructor( private http: httpClient ) { }

  chuckUrl = 'https://api.chucknorris.io/jokes/random';

  getChuck() {
      return this.http.get<DataModel>(this.chuckUrl);
  }

}

data.model.ts

export class DataModel {
    public category: any;
    public icon_url: String;
    public id: String;
    public url: String;
    public value: String;
}

data.component

import { Component,OnInit } from '@angular/core';
import { Dataservice } from './data.service';
import { DataModel } from './data.model';

@Component({
    SELEctor: 'app-data',templateUrl: './data.component.html'
})

export class ApPDAta implements OnInit {
    constructor(private dataservice: DataservicE) { }

    joke: DataModel;

    ngOnInit() {

        this.dataservice.getChuck()
            .subscribe(
                (data: DataModel ) => {
                  if (data.category != 'explicit') {
                    this.joke = data;
                  } 
                }
            ); 
    }

}

<div>{{ joke?.value }}</div>