@Nullable
  public static TreeStructureProvider get(@Nullable Project project) {
    if (project == null || project.isDisposed() || project.isDefault()) return null;
    TreeStructureProvider provider = project.getUserData(KEY);
    if (provider != null) return provider;
    provider = new CompoundTreeStructureProvider(project);
    project.putUserData(KEY, provider);
    return provider;
  }
@Nullable
  public static TreeStructureProvider get(@Nullable Project project)