to reproduce:
1.) download & extract XnView MP 0.39
2.) Create an 'xnview.ini' in the same extracted program folder(s)
3.) Run the program, and "Import Clipboard" an image
4.) Click on 'Spotlight' tool, don't draw anything as yet
5.) Now enter fullscreen mode (using middle click)
6.) With 'Spotlight' toolbar present in fullscreen, now draw a few spotlights and press 'undo' button
7.) Program crash!!
EDIT:-
The same issue happens with the 'Crop' toolbar-function in Fullscreen view. So open any image in XnView MP viewer, press crop (to show crop toolbar) and then enter Fullscreen and draw a crop selection and press 'crop' button = program crash. Program crash happens after pressing crop button.
I just believe that there should not be a program crash in those places...It is actually great (without the bug

Windows 7 sp1