Well...at least it's repeatable. It might actually be a Qt bug, but I thought I'd mention it just in case it's something easily fixed.
Run a report and select Image Files from the left column.
--- If you have a list of images and one of those images is used "0" (zero) times, delete that image by highlighting it on the file list at the right and press the delete key then confirm the delete. The file name stays in the list although the image file itself is deleted.
--- If you then try and sort the list of files by clicking on one of the header columns then Sigil will crash... because it can't sort a file that isn't really there.
Of course the report can be "refreshed" or re-run which will remove the deleted filename from the list and avoid either of these issues... but, I figured it would be better if the program didn't crash in the first place.
(Windows 10, 64)
Merry Christmas!
edit: OH Look - I have exactly 1234 posts!!! I'm not going to post for at least a week now!!