@Nullable
  private String verifyInnerClassDestination() {
    PsiClass targetClass = findTargetClass();
    if (targetClass == null) return null;

    for (PsiElement element : myElementsToMove) {
      if (PsiTreeUtil.isAncestor(element, targetClass, false)) {
        return RefactoringBundle.message("move.class.to.inner.move.to.self.error");
      }
      final Language targetClassLanguage = targetClass.getLanguage();
      if (!element.getLanguage().equals(targetClassLanguage)) {
        return RefactoringBundle.message(
            "move.to.different.language",
            UsageViewUtil.getType(element),
            ((PsiClass) element).getQualifiedName(),
            targetClass.getQualifiedName());
      }
      if (element.getLanguage().equals(Language.findLanguageByID("Groovy"))) {
        return RefactoringBundle.message("dont.support.inner.classes", "Groovy");
      }
    }

    while (targetClass != null) {
      if (targetClass.getContainingClass() != null
          && !targetClass.hasModifierProperty(PsiModifier.STATIC)) {
        return RefactoringBundle.message("move.class.to.inner.nonstatic.error");
      }
      targetClass = targetClass.getContainingClass();
    }

    return null;
  }