Debug window has no X (close) button

I am using Qt Creator in my C++ lectures. All pupils like the tool, but
there is a little distraction:

All windows my be closed with the little X in the top right corner –
actually it is "all but one": if you change to debug (which may happen
by accident) there is no X and a lot of confusion. I would like to
suggest such an X to switch back to edit mode.