private @Nullable String doFormatAvailableUndoRedoAction(@Nullable FileEditor editor, boolean isUndo) {
    ClientState state = getClientState(editor);
    if (state == null) {
      return null;
    }
    Collection<DocumentReference> refs = getDocRefs(editor);
    if (refs == null) return null;
    if (isUndo && state.myMerger.isUndoAvailable(refs)) return state.myMerger.getCommandName();
    return getStackHolder(state, isUndo).getLastAction(refs).getCommandName();
  }
private @Nullable String doFormatAvailableUndoRedoAction(@Nullable FileEditor editor, boolean isUndo)