code: @Override
      public String getFirstValidationGroupFromStack()
      {
          if (_validationGroupsStack != null && !_validationGroupsStack.isEmpty())
          {
              return _validationGroupsStack.getFirst(); // top-of-stack
          }
          return null;
      }
docs: Gets the top of the validationGroups stack.
  @return
  @since 2.0