小编use*_*113的帖子

在服务中发出 HTTP 请求时，我收到此响应：

TypeError: Cannot read property 'length' of null
    at eval (http.js:123)
    at Array.forEach (<anonymous>)
    at HttpHeaders.lazyInit (http.js:117)
    at HttpHeaders.init (http.js:265)
    at HttpHeaders.forEach (http.js:368)
    at Observable.eval [as _subscribe] (http.js:2172)
    at Observable.subscribe (Observable.js:162)
    at eval (subscribeToObservable.js:16)
    at subscribeToResult (subscribeToResult.js:6)
    at MergeMapSubscriber._innerSub (mergeMap.js:127)
ALERT!!!!! 

我在加载组件时订阅了 HTTP 请求：

export class TasksComponent implements OnInit {
  currentUser:any;
  username:string=null;
  constructor(private usersService:UsersService) {}
  ngOnInit() {
    this.username=localStorage.getItem('currentUsername');
    console.log(this.username);
    this.usersService.getUserByUsername(this.username)
      .subscribe(data=>{
        console.log(data);
        this.currentUser=data;
      },err=>{
        console.log(err);
        console.log("ALERT!!!!! ");
      })
  }
}

用户服务：

//for getting a user by its username …