I have looped through the _SCREEN object forms collection, I assign the form I want to a variable (loForm) and then try to set the focus. I also tried DO loForm.activate(), but that came up with the same error.
I don't know if you've seen the other thread with Hilmar Zonnefeld but he suggested using .show(). I tried that and set the focus to a control on the form and got the strangest behaviour of having a control with the focus (it was a button...pressing return ran the click event) but the form had no focus. It's only important because I have a timer control that is trying to trap CTRL-N...if the form does not have focus then it doesn't work.
My colleagues have warned against using modeless forms...they also warned against using combo boxes!!!! I'm not used to using modal forms for everything and I certainly will not accept life without combo's. But that's a different story.