I had a smelter running, and I clicked to cancel the last item in the queue (3rd position), just as the smelter was finishing its current item.

I suspect it may have ended up trying to delete a null item from that queue slot, and thus crashed.

That might be the case - I just tried doing this myself and couldn't get it to happen. But there might be a way that the timing is just right for it to throw a null-ref. I'll probably at least add in some extra checks around canceling things in the queue just to be safe.

Thanks!