title: failed to run unit test for component that extends a base class in angular
body: <p>I am trying to write unit test for a component that extends a base class.</p>
  
  <p><strong>component.ts</strong></p>
  
  <pre><code>   export class DesktopPresentationAreaComponent extends AbstractPresentationComponent {
              constructor() {
                  super(store, webcast);
              }
     }
  </code></pre>
  
  <p><strong>base class</strong></p>
  
  <pre><code>export abstract class AbstractPresentationComponent {
      constructor(protected store: MainStorageService,
                  protected webcast: WebcastService) {
          this.store.get('state').subscribe();
      }
  </code></pre>
  
  <p>when I run 'ng test' it shows following error.</p>
  
  <pre><code>TypeError: Cannot read property 'subscribe' of undefined in AbstractPresentationComponent
  </code></pre>
  
  <p>how can I solve this issue?</p>