如何为使用NgbDropdown的组件编写测试？

Question

我有一个使用NgbDropdown的组件，并且正在为我的组件编写组件测试。我遇到了错误。

Failed: Cannot read property 'placement' of undefined
    TypeError: Cannot read property 'placement' of undefined
        at new NgbDropdown node_modules/@ng-bootstrap/ng-bootstrap/dropdown/dropdown.js:130:1)
        at createClass node_modules/@angular/core/esm5/core.js:12423:1)
        at createDirectiveInstance node_modules/@angular/core/esm5/core.js:12266:22)
        at createViewNodes node_modules/@angular/core/esm5/core.js:13727:38)
        at callViewAction node_modules/@angular/core/esm5/core.js:14161:1)
        at execComponentViewsAction node_modules/@angular/core/esm5/core.js:14070:1)
        at createViewNodes node_modules/@angular/core/esm5/core.js:13755:1)
        at createRootView node_modules/@angular/core/esm5/core.js:13616:1)
        at callWithDebugContext node_modules/@angular/core/esm5/core.js:15041:26)
        at Object.debugCreateRootView [as createRootView] node_modules/@angular/core/esm5/core.js:14324:1)

这是我的组件规格

describe("SomeComponent", () => {
  beforeEach(async(() => {

    TestBed.configureTestingModule({
      declarations: [
      ],
      imports: [
        NgbDropdownModule,
        HttpClientTestingModule,
      ],
      providers: [
        {provide: Router},
        {provide: NgbModal},
        {provide: NgbDropdownConfig}

      ],
      // schemas: [NO_ERRORS_SCHEMA]
    }).compileComponents();

    let fixture = TestBed.createComponent(SomeComponent);
    let component = fixture.componentInstance;
    let httpMock = TestBed.get(HttpTestingController);
  }));

  it("should create the app", () => {

    const fixture = TestBed.createComponent(AppComponent);
    const app = fixture.debugElement.componentInstance;
    expect(app).toBeTruthy();
  });
});

相关组件html

 <div ngbDropdown>
    <button ngbDropdownToggle>Choices</button>
    <div ngbDropdownMenu>
      <a>item choice</a>
    </div>
  </div>