code: protected Stage determineStage(final Actor actor) {
          Stage stage = actor == null ? null : actor.getStage();
          if (stage == null) {
              LmlTag ancestorTag = getParent();
              while (ancestorTag != null && stage == null) {
                  final Actor ancestor = ancestorTag.getActor();
                  if (ancestor != null) {
                      stage = ancestor.getStage();
                  }
                  ancestorTag = ancestorTag.getParent();
              }
          }
          return stage;
      }
docs: Utility method. Allows to extract a {@link Stage} instance of either the passed actor or any of its parents.
  
  @param actor can be null. Its stage can be null.
  @return a {@link Stage} instance if any of the actors in hierarchy is added to one.