Hi Sijo,
You can put a restriction by checking whether the field is of type 'Label' like
IF iv_element_name = 'label'. Then the error won't come.
Also, you were saying that you are trying to give tooltip. Are you giving tooltip for the contents or for the heading?. If for contents, why can't you go with get_p 'WHEN if_bsp_wd_model_setter_getter=>fp_tooltip' rather than changing the iterator class?.
Thanks,
Faisal