I installed version 3.28.1-0ubuntu3 of gnome-session-bin gnome-session-common and ubuntu-session from bionic-proposed, and the problem went away.
(When I upgraded from 16.04 to 18.04, do-release-upgrade stopped with an error, so I did the rest of the upgrade manually.)
I installed version 3.28.1-0ubuntu3 of gnome-session-bin gnome-session- common
and ubuntu-session from bionic-proposed, and the problem went away.
(When I upgraded from 16.04 to 18.04, do-release-upgrade stopped with an error,
so I did the rest of the upgrade manually.)