public @Nullable String checkValidity() {
    if (!myFile.isValid()) {
      return "Invalid file";
    }
    final DomFileElementImpl<DomElement> fileElement = myManager.getFileElement(myFile);
    if (!equals(fileElement)) {
      return "file element changed: " + fileElement + "; fileType=" + myFile.getFileType();
    }
    return null;
  }
public @Nullable String checkValidity()