docs: def get_tree(self, work_item_id):
          """Gets XML tree of the workitem."""
          try:
              __, tcid = work_item_id.split("-")
          except ValueError:
              logger.warning("Couldn't load workitem %s, bad format", work_item_id)
              self._cache[work_item_id] = InvalidObject()
              return None
  
          path = os.path.join(self.test_case_dir, self.get_path(tcid), work_item_id, "workitem.xml")
          try:
              tree = etree.parse(path)
          # pylint: disable=broad-except
          except Exception:
              logger.warning("Couldn't load workitem %s", work_item_id)
              self._cache[work_item_id] = InvalidObject()
              return None
          return tree