我正在测试一个已经注入了服务的组件。正如我所看到的，问题是它没有进入ngOnDestroy（）。我明确要求测试destroy（），每次测试后我都会收到错误消息"无法取消订阅未定义的"。可能是什么问题？
组件：

//...
    subscription: Subscription;

    constructor(private readonly router: Router,
                private dialog: MatDialog,
                private readonly carService: carService) {
    }

    ngOnInit(): void {
        this.cars$ = this.carService.getCars();
        this.subscription= this.cars$.subscribe(cars=> {
            this.cars= cars;
        });
    }

    ngOnDestroy() {
        this.subscription.unsubscribe();
    }

试验：

//...
  beforeEach(async () => {
        await TestBed.configureTestingModule({

            providers: [
                {provide: MatDialog, useClass: dialogMock},
                {provide: Router, useValue: routerMock},
                {provide: APP_BASE_HREF, useValue: '/cars'}
            ]
        }).compileComponents();

        component = TestBed.createComponent(CarComponent).componentInstance;
        service = TestBed.inject(CarService);
        service.setProjects([firstCar, secondCar]);
        component.ngOnInit();
    });

    it('test', () => {
        expect(component).toBeInstanceOf(CarComponent);
    });