@Nullable
  public @Nls(capitalization = Nls.Capitalization.Sentence) String lookupError(XmlName xmlName) {
    final ClassProvider provider = myCustomElements.get(xmlName);
    return provider == null ? null : provider.getError();
  }
@Nullable
  public @Nls(capitalization = Nls.Capitalization.Sentence) String lookupError(XmlName xmlName)