错误类型错误:无法读取未定义的属性“id”

我得到了一个 错误类型错误:无法读取未定义的属性“id” 当我导航到组件时。在调试代码之后,我发现我的错误在哪里,但我没有找到正确的方法来解决它。也许你能帮我?

下面是我的.ts文件:

@Component({
  selector: 'app-forum-thread',
  templateUrl: './forum-thread.component.html',
  styleUrls: ['./forum-thread.component.css']
})
export class ForumThreadComponent implements OnInit {
  @Input() theme: Theme;
  threads: Observable<Thread[]>;

constructor(
  private route: ActivatedRoute,
  private location: Location,
  private themeService: ThemeService,
  private threadService: ThreadService
) { }

ngOnInit() {
  this.getTheme();
  this.getThreads();
}

getTheme(): void {
  this.themeService.getTheme(this.route.snapshot.paramMap.get('theme'))
    .subscribe(theme => this.theme = theme);
}

// Error caused by this method..
getThreads(): void {
  this.threads = this.threadService.getThreads(this.theme.id);
}

以及相应的.html文件:

<div *ngIf="theme">
  <h2>{{ theme.name | uppercase }}</h2>

  <table class="Threads">
     <tr>
       <th>Id</th>
       <th>Date</th>
       <th>Title</th>
       <th>Views</th>
       <th>Thanks</th>
       <th>Comments</th>
     </tr>
     <tr *ngFor="let thread of threads | async">
       <a routerLink="/forum-message/{{theme._id}}/{{thread._id}}">
         <td><span class="badge">{{thread._id}}</span></td>
       </a>
       <td>{{thread.createdAt | date}}</td>
       <td>{{thread.title}}</td>
       <td>{{thread.numberOfViews}}</td>
       <td>{{thread.numberOfThanks}}</td>
       <td>{{thread.numberOfComments}}</td>
     </tr>
   </table>  
 </div>

我试图再次调用路由快照,结果成功了。 但我不喜欢两次调用路由快照,所以也许还有其他方法?

getThreads(): void {
  this.threads = this.threadService.getThreads(
    this.route.snapshot.paramMap.get('theme'));
}

提前谢谢你的帮助!