I am experiencing the same exact issue, it seems to happen when you handle the playlist in any way and you don't give it time to "buffer" in a way.
For example if I start playing a track and immediatly start click another part of the track WACUP will crash, if I give it a couple of seconds while it plays the track or do it while the track is paused it will not.
This crash also seems to just close the program without the usual message of it sending a crash report to the wacup website so it's likely the reason why you don't see the report, it has been happening a lot and most of those times it never showed the crash report message.