how to handle sy-ucomm in generated screen/dynpro

Hi,

like the topic says, I have a problem to handle

the user-input in or better, after the first screeen!

I think, the best Info is the code itself:

SELECTION-SCREEN BEGIN OF screen 100 as window.
SELECT-OPTIONS:  s_kunnr  for wa_besuche-kunnr,
                 s_bzirk  FOR wa_besuche-bzirk,
                 s_vtweg  FOR wa_besuche-vtweg,
                 s_vkgrp  FOR wa_besuche-vkgrp,
                 s_dat    FOR sy-datum DEFAULT '20070101' TO '20071231'.
SELECTION-SCREEN END   OF screen 100.


call screen 100.


datumuntergrenze = s_dat-low.
datumobergrenze  = s_dat-high.

perform daten_selektieren_generieren.


* Here I want to handle sy-ucomm (if the user doesn't 
* pushed the EXIT/CANCEL/BACK-Buttton in screen 100,
* the following screen 200 should be called)
* --------> BUT SY-UCOM IS EMPTY

call screen 200.

I would be very nice, if somebody can

help me with this sh...!