Comment 17 for bug 1571564

Correction, downgrading to plasma-desktop-data_5.5.4-0ubuntu1_all.deb made the pop-up go away, and I could start plasmashell again. However, my panel was still messed up and clicking kickoff produced an error.

I then noticed that plasma-desktop was not installed (showed rc in dpkg -l), so I manually installed plasma-desktop_5.5.4-0ubuntu1_amd64.deb and now everything seems fine after logging out and in again.