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
Gets XML tree of the workitem.