无法读取未定义的属性取消订阅:Angular2测试

我正在尝试测试一个具有ngOnDestroy((方法的组件，该方法具有所有的unsubscribe((调用。但是，在测试时，当我运行测试文件(规范文件(时，它会给我一个错误，说：

cannot read property 'unsubscribe' of undefined

我已经在我的测试文件中完成了以下导入：

import { Subscription }   from 'rxjs/Subscription';

所以我认为这应该足以获得Subscription中的所有方法，但错误仍然存在。此外，我尝试将"Subscription"添加到测试文件的"imports"、"declarations"one_answers"providers"列表中，但错误仍然存在。

下面是一个代码片段：

//component
import { Component, OnInit, OnDestroy } from '@angular/core';
import { Subscription }   from 'rxjs/Subscription';
import {
 NinjaService } from '../../../../ninja.service';

 @Component({
 selector: 'ninja-files',
 templateUrl: './ninja-files.component.html',
 styleUrls: ['./ninja-files.component.css']
})
 export class ninjaFilesComponent implements OnInit, OnDestroy {
 showNinjaFiles: boolean = true;
 addFileSubscription: Subscription;
 constructor(private ninjaService: NinjaService) {
 ........
}
 ngOnInit() {
  this.addFileSubscription = this.NinjaService.AddedFile$
  .subscribe((fileFormat: FileFormat) => {
  console.log('File being added: ' + fileFormat.fileName); }
  ngOnDestroy() {
  this.addFileSubscription.unsubscribe();
}
}

然后我有这个组件的测试文件如下：

import { Component, OnInit, OnDestroy } from '@angular/core';
import { Subscription }   from 'rxjs/Subscription';
import {
 NinjaService } from '../../../../ninja.service';
import { TestBed, async, fakeAsync ,ComponentFixture, } from '@angular/core/testing';
import { DebugElement }    from '@angular/core';
import { By }              from '@angular/platform-browser';
import {} from 'jasmine';
describe('Component: NinjaFiles', () => {
  let fixture: ComponentFixture<ninjaFilesComponent>;
  let component: ninjaFilesComponent;
  let ninjaServiceStub;
beforeEach( async(() => {
 //stub NinjaService
 let ninjaServiceStub= {};
 fixture = TestBed.configureTestingModule({
 declarations: [ ninjaFilesComponent],
}).createComponent(ninjaFilesComponent);
 component = fixture.componentInstance;
}));
it('Page Title', async(() => {
 let pageTitle = fixture.debugElement.query(By.css('.page-title'));
 expect(pageTitle).toBeTruthy();
}));
 it('Counting the files', () => {
  let fileCount= fixture.debugElement.query(By.css('.file-count'));
  expect(fileCount).toBeTruthy();
});

当我运行上面的测试代码时，它会给我一个错误，说"无法读取未定义的属性‘unsubscribe’"(这意味着它无法定义我在组件类中定义的订阅对象‘addFileSubscription’。

有人能提出一个变通办法吗？