@Nullable
  private static XmlFile getRngSchema(XmlFile file) {
    final RngElementDescriptor descriptor = getRootDescriptor(file);
    if (descriptor == null) return null;

    final PsiElement element = descriptor.getDeclaration();
    final XmlFile schema = PsiTreeUtil.getParentOfType(element, XmlFile.class);
    if (schema == null) return null;
    return schema;
  }
@Nullable
  private static XmlFile getRngSchema(XmlFile file)