code: @Nonnull
    public final ITEMTYPE createChildItem (@Nullable final DATATYPE aData)
    {
      // create new item
      final ITEMTYPE aItem = m_aFactory.create (thisAsT ());
      if (aItem == null)
        throw new IllegalStateException ("null item created!");
      aItem.setData (aData);
      internalAddChild (aItem);
      return aItem;
    }
docs: Add a child item to this item.
  
  @param aData
  the data associated with this item
  @return the created TreeItem object or <code>null</code> if the ID is
  already in use and bAllowOverwrite is false