undefined is not an object (evaluating 'config.forEach') - Angular2 Route Test

i'm a complete newbie in Angular2 especially in Testing. I've searched the web for a solution but there is so much old code examples that doesn't work anymore. So I came here to ask you.

When i ran my test with "karma test". I get the following error:

Error: Error in ./NavbarItemComponent class NavbarItemComponent - inline template:1:1 caused by: undefined is not an object (evaluating 'config.forEach') in config/karma/karma-test-shim.js (line 20623)

app.component.spec.ts

import { TestBed } from '@angular/core/testing';
import { AppComponent } from './app.component';
import { NavbarVerticalComponent } from './navbar-vertical/navbar-vertical.component';
import { NavbarItemComponent } from './navbar-item/navbar-item.component';
import { Router } from '@angular/router'
import { RouterTestingModule } from '@angular/router/testing';

class MockRouter extends Router {
  navigate = jasmine.createSpy('navigate');
  routerState: RouterState;
}

describe('App', () => {
  beforeEach(() => {
    TestBed.configureTestingModule({ 
    declarations: [AppComponent, NavbarVerticalComponent, NavbarItemComponent ],
    providers: [
      { provide: Router, useClass: MockRouter  }
    ],
    imports: [RouterTestingModule]
    });
  });

  it ('should work', () => {
    let fixture = TestBed.createComponent(AppComponent);
    expect(fixture.componentInstance instanceof AppComponent).toBe(true, 'should create AppComponent');
  });

});