Destroy the Reference
When the AddressForm is closed, it signals a user-defined event:
SIGNAL event(u_detail_form_closed)
The MainForm has an event routine which handles this event by setting the reference to the AddressForm to *NULL:
EVTROUTINE handling(#AddressForm.u_Detail_form_closed)
SET_REF com(#AddressForm) to(*null)
ENDROUTINE
This means that when AddressForm closes, it is removed from memory. The form itself as well as the button and the fields on it all disappear from the system.