docs: public Collection<TopLevelItem> getAllItems() {
  
          if (this instanceof ViewGroup) {
              final Collection<TopLevelItem> items = new LinkedHashSet<>(getItems());
  
              for(View view: ((ViewGroup) this).getViews()) {
                  items.addAll(view.getAllItems());
              }
              return Collections.unmodifiableCollection(items);
          } else {
              return getItems();
          }
      }