code: private Map<String, TreeItemIdNode> createCustomIdNodeMapping() {
  		TreeItemIdNode custom = getCustomTree();
  		if (custom == null) {
  			return Collections.EMPTY_MAP;
  		}
  
  		Map<String, TreeItemIdNode> map = new HashMap<>();
  		processCustomIdNodeMapping(map, custom);
  		return Collections.unmodifiableMap(map);
  	}
docs: Create the map between the custom item id and its node.
  
  @return the map between an item id ands its custom node