Calling a popup dialog box and trigger workflow from a PO change BADI

We've a PO change BADI that is is called when one uses PO create of change transaction. In this BADI we are triggering a workflow in the background.

Now User is asking me to display a diaglog box when wt exceededs a certain limit.

When I add code for displaying dialog box in the BADI, I get the error message "Control Error".

It works fine w/o the dialog box.

Any clue why I am getting this error message?