Angular 2 Cannot read property 'subscribe' of undefined TypeError: Cannot read property 'subscribe' of undefined

I have tried many propositions on fixing this error here on stackoverflow, but now I just need to ask as I have spent many hours with literally getting nowhere now.

I have this simple Service:

constructor(private http: Http, private tokenResolver: TokenResolver) {}

public getEventHistory(): Observable<heatmapElement[]> {

  this.tokenResolver.getToken(true).subscribe(token => {
    var headers = new Headers();
    headers.append('Authorization', 'Bearer ' + token);
    headers.append('Content-Type', 'application/json');

    this.result = this.http.get(this.restApi, new RequestOptions({headers: headers}))
        .map((res: Response) => res.json());
  });

  return this.result as Observable<heatmapElement[]>;
}

I want to get the data using :

public demoData: heatmapElement[] =[];

getEventHistory(): void {
  this.eventHistoryService.getEventHistory()
                          .subscribe(data => this.demoData = data,);
}

This creates a error that I just cant seem to fix:

EXCEPTION: Uncaught (in promise): Error: Error in http://localhost:5555/app/dashboard/dashboard.html:13:8 caused by: Cannot read property 'subscribe' of undefined TypeError: Cannot read property 'subscribe' of undefined

I would be very grateful for help, thank you