code: private void normalizeSelectedCategories() {
  
          Collection<String> normalizedCategories = new ArrayList<String>(m_selectedCategories.size());
          for (CmsTreeItem item : m_categories.values()) {
              if (item.getCheckBox().isChecked()) {
                  normalizedCategories.add(item.getId());
              }
          }
          m_selectedCategories = normalizedCategories;
  
      }
docs: Normalize the list of selected categories to fit for the ids of the tree items.