code: public void back(Class<? extends Controller> controllerClass) {
          String toLocationId = controllerClass == null ? null : controllerClass.getName();
          navigateBackToLoc(toLocationId);
      }
docs: Navigates back. If current location is null it doesn't take any effect. When controllerClass
  is null, navigate to the very first location and clear all history prior to it, otherwise
  navigate to location with given locationId and clear history prior to it. Then a
  {@link NavigationManager.Event.OnLocationBack} event will be raised.
  
  @param controllerClass the controller class type