TypeError: Cannot read property 'instance' of undefined

Here is the method i am trying to test:

showSnackbar(): void {
    if (this.modifiedReferences.length) {
      const snackbar = this.snackbarService.open({
        message: '',
        type: 'success',
        duration: 10000,
        actionText: 'SHOW DETAILS'
      });
      snackbar.instance.actionClicked.subscribe(() => {
        this.dialogService.modal(ModifiedComponent);
      });
    }
}

and here is the test:

  it('should call the showSnackbar method', () => {
    const showSnackbarSpy: jasmine.Spy = spyOn(TestBed.inject(SnackbarService), 'open');

    component.showSnackbar();

    expect(showSnackbarSpy).toHaveBeenCalled();
  });

When i try to test it displays zn error saying that it can't reag property 'instance' of undefined