hendrik on 27 Jan, 2020 11:25 AM
Please shutdown both beaTunes and Music.app, then start beaTunes and synchronize again (File -> Synchronize). Perhaps it disappears then. Hopefully that fixes it.
There have been reports that for some reason Music.app sometimes still reports old playlist as existing when in fact they have been deleted.
Regarding that NullPointerException—that occurred when you tried to create a new playlist in a folder that does not exist. This is something that should not be possible, therefore the non-informative error message.