Hi,
From what I understood, you are facing following issue:-
If the user does not enter value in a mandatory field, he is taken to another screen,
He MUST enter data and hit BACK button.
This takes him to main screen of ISQ21
You are (or were) stuck with Control not found error
User ignores mandatory field and clicks BACK
He gets SAP error (mostly pop-up with Yes/No)
Then do you come back to main screen of ISQ21?
I think you can script in something like this.
if exists (check control id of the mandatory field on screen)
(assume user has entered value)
Press BACK
(assuming pop-up is appearing)
If exists( control id of Yes button)
Push
(hopefully you should be on ISQ21 screen)
Press BACK as needed.
Let me know if the assumptions are correct and whether you already tried what I've written.
Regards
Abhi