@Nullable
    private ProjectFileNode getSingleDirectory(ProjectFileNode parent) {
      List<ProjectFileNode> children = model.getChildren(parent);
      ProjectFileNode child = children.size() != 1 ? null : children.get(0);
      return child != null && child.getVirtualFile().isDirectory() ? child : null;
    }
@Nullable
    private ProjectFileNode getSingleDirectory(ProjectFileNode parent)