docs: public CoreActivity findActivity(String activityId) {
      CoreActivity localActivity = getChildActivity(activityId);
      if (localActivity!=null) {
        return localActivity;
      }
      for (CoreActivity activity: getActivities()) {
        CoreActivity nestedActivity = activity.findActivity(activityId);
        if (nestedActivity!=null) {
          return nestedActivity;
        }
      }
      return null;
    }