private static @Nullable PsiElement getIdentifier(@Nullable PsiReferenceExpression expression) {
        if (expression == null) return null;
        PsiIdentifier[] identifiers = PsiTreeUtil.getChildrenOfType(expression, PsiIdentifier.class);
        if (identifiers == null || identifiers.length != 1) {
          return null;
        }
        return identifiers[0];
      }
private static @Nullable PsiElement getIdentifier(@Nullable PsiReferenceExpression expression)