Either with a alternative syntax:
_control ctrlSetFocus boolean

Obviously the result would be:
for the given control true would set focus, false would kill focus.

Or:
Allow ctrlSetFocus to focus on a null control.

Currently you have no way to kill focus from a control via scripting unless you ctrlSetFocus on some other existing control (which can be non-desirable in some sitiations), and you can not focus on null control to do that either.