Mock a library directive from angular

I want to unit test a component which uses a directive of a library. My code looks like this:

// component.spec.ts
@Directive({
    selector: 'mwlFlatpickr',
})
class mwlFlatpickr implements OnDestroy{
  @Input('mwlFlatpickr') public x: any;
  @Input('convertModelValue') public y: boolean;
  @Input('dateFormat') public z: string;
  public ngOnDestroy(): void {}
}

describe('Component', () => {
  let component: Component;
  let fixture: ComponentFixture<Component>;

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [
        mwlFlatpickr
      ],
      imports: [
         // other modules
      ],
      providers: [
        // my services
      ]
    })
      .compileComponents();
  }));
  beforeEach(() => {
      fixture = TestBed.createComponent(CreateTaskComponent);
      component = fixture.componentInstance;
  });

html:

// component.html
 <input type="text" class="floating-label-field" mwlFlatpickr [convertModelValue]="true" dateFormat="d.m.Y" formControlName="dueDate">

My Problem is: the component uses the real mwlFlatpickr, which throws an error in it's onDestroy (the component instance is not defined, but that is out of the scope of my tests)

 TypeError: Cannot read property 'destroy' of undefined
    at FlatpickrDirective.ngOnDestroy (angularx-flatpickr.js:330)
    at executeOnDestroys (core.js:14123)
    at cleanUpView (core.js:14030)
    at destroyViewTree (core.js:13799)
    at destroyLView (core.js:13981)
    at RootViewRef.destroy (core.js:14836)
    at ComponentRef$1.destroy (core.js:33986)
    at ComponentFixture.destroy (testing.js:423)
    at UserContext.<anonymous> (component.spec.ts:160)
    at ZoneDelegate.invoke (zone-evergreen.js:364)

the error occurs in the clean up, so the test doesn't matter. Imagine the itto just be:

 it('should not throw an error', () => {
    expect(component).toBeDefined();
 });

I'm a little dumbfounded. All tutorials and topics I found solved a mocked directive in that way (and it works with pipes just fine) The directive class is declared as:

export declare class FlatpickrDirective implements AfterViewInit, OnChanges, OnDestroy, ControlValueAccessor

natrually, I tried to change the name to that and implemented the other implements as well - same error, the component does not use my mockDirective