What is your computer setup? I had the same problem when I tried to make levels on my convertible tablet. Integrated graphics don't do well with 3d. Its not just gamestudio I can't use ultimate unwrap either. I can do bryce renders though which is odd.
So.. if this is your problem the only thing you can do is kill any resource hogs on your system, usually the antivirus.
sometimes zooming in and out can make the box reappear, sometimes selecting the block from the side bar works. The box isn't gone, it just isn't visible on your screen. build the level and it will be there.
I don't think I have ever had this problem on a PC with a discrete card.