@Override
    public @Nullable Comparable<PsiFileNode.ExtensionSortKey> getTypeSortKey() {
      String extension = getVirtualFile().getExtension();
      return extension == null ? null : new PsiFileNode.ExtensionSortKey(extension);
    }
@Override
    public @Nullable Comparable<PsiFileNode.ExtensionSortKey> getTypeSortKey()