Thanks but that does not work either. Maybe it's best to add more information.

I'm working with AccountMate LAN 505. From the menu I open a formset and make the main form active. But I want only one instance of the formset to be active (single instance). So in the init of the formset I look for another main form. But your code and my code closes it's self even it's the first form. I'll try the load of the formset.
TIA
John