GeolocationService null error in qs. 3/4.

public class GeolocationService extends Service {
    private IBinder binder = new LocalBinder();

    public class LocalBinder extends Binder {
        public GeolocationService getService() {
            return GeolocationService.this;
        }
    }

    @Override
    public IBinder onBind(Intent intent) {
      return binder;
    }

    //Client methods
    public int getLatitude() { return getLat(); }    
    public int getLongitude() { return getLng(); }
}

public class MainActivity extends Activity {
  int latitude;
  int longitude;
  boolean isBound;
  Button updateLocationButton;
  private GeolocationService mGeolocationService;

  ServiceConnection serviceConnection = new ServiceConnection(){
    public void onServiceConnected(ComponentName name, IBinder binder) {
      isBound = true;
      GeolocationService.LocalBinder localBinder = (GeolocationService.LocalBinder) binder; 
      mGeolocationService = localBinder.getService();
    }
    public void onServiceDisconnected(ComponentName name) {
      isBound = false;
    }  
  };  

  @Override
  public void onCreate(Bundle savedInstanceState) {
    setContentView(R.layout.activity_main);

    updateLocationButton = (Button) findViewById(R.id.update_button);
    updateLocationButton.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v){

      }
    });
  } 

  @Override
  public void onStart() {
    Intent i = new Intent(this, GeolocationService.class);
    bindService(i, serviceConnection, Context.BIND_AUTO_CREATE);
  }

  @Override
  public void onStop() {
    if (isBound) {
      unbindService(serviceConnection);  
      isBound = false;
    }
  }
}