sorry to hear that, TPReal. we have been using the designer internally but have not run into that problem. do you remember any more details about it? was this the first level you designed? what was the change you had made, which caused the walls to vanish? did you have all types of objects (down walls, right walls, white buttons, gray buttons) on the level? and out of these, only the walls disappeared?
if it happens again, or you can make it happen, please send more details to support@wildnoodle.com.
thanks,
brian