Here is the mentioned save file.
Well, that's a rather nasty bug.
There's a dying reaper (see attached), which dies when turn ends... and the game tries to convert it to corpse... but in the process it checks some tiles outside of map boundaries, where it fails on null pointer exception.
I don't know what it is, could be:
- a bug in oxce, yankes has been doing some changes there recently
- a wrong merge
- an incomplete merge (I noticed vanilla has some more conditions around calling this function than oxce)
- a bug in vanilla
I'll report to Yankes, he'll probably deal with it better than I can.
I also don't know how to avoid this bug in your save... killing all enemies in debug mode won't help either.
If you have an earlier save... try reloading... and don't kill a reaper if it's at the map border... wait until it is at least one tile away from border.
EDIT: my feeling is that lastPosition of the reaper should not be [59,24,0] but [58,23,0]... someone who knows more should confirm tho