static @Nullable PsiElement resolveReference(@Nullable XmlAttribute ref) {
    if (ref != null) {
      XmlAttributeValue value = ref.getValueElement();
      if (value != null) {
        PsiReference[] references = value.getReferences();
        if (references.length > 0)
          return references[0].resolve();
      }
    }
    return null;
  }
static @Nullable PsiElement resolveReference(@Nullable XmlAttribute ref)