def get_index(self, index_struct_id: Optional[str] = None) -> BaseIndex:
        """Get index from index struct id."""
        if index_struct_id is None:
            index_struct_id = self._root_id
        return self._all_indices[index_struct_id]
Get index from index struct id.
llama-index-core/llama_index/core/indices/composability/graph.py