Been getting this error for a while. I've emailed Marcus about it, but I'm still experiencing the error several builds later. I'm starting this thread so everyone experiencing the error can share the details, and Marcus can get it tracked down.
I have "Delete episodes after listened" enabled. With some episodes, after completion, I get an IllegalArgumentException null error, and the episode does not delete. The error prevents continuation of the remainder of the playlist until I manually handle the error, and also results in undeleted episodes that I must later track down.
When I later attempt to delete the episodes, a straightforward delete does not work (I get another error). Through experimentation, I've found that it's possible to delete the episode with the following procedure:
1) Navigate to the episode
2) Go to the episode details page
3) Rewind the episode by any increment
4) Delete the episode
This works whether or not the episode in question is playing, and even works if some other episode is playing in the background.
On a side note, I've noticed that when I open PT, these complete-but-undeleted episodes do not show up in the episode list until the feed gets re-downloaded. This makes these rogue episodes difficult to find and delete. If you can't fix the error, perhaps you could check to see if the file has actually been deleted before the episode is removed from the episode list.
Sorry for the long post, but this error has been bothering me for the past month or so.