I had the same situation. In fact I posted a thread here today in the codemine section. I found another work-around. If you issue thisform.hide() immediately before launching your modal form. The main screen remains closable.

Strangely, thisform.show(), does not work.

However, I like Fred's solution better. It gives you the option of seeing your parent form.

Maybe this will help someone figure out what is going on.