1 - Finally had a successful "clean" shutdown. Plasma kept crashing on me and one time, it didn't. The next time I rebooted, I didn't get this prompt.
2 - I went into the settings and set "start with a new session" as my choice, rather than the default of "restore previous session". I forget where exactly this setting is in the System Settings.
I like poetry, long walks on the beach and poking dead things with a stick.
Thanks. Haven't had any unclean shutdowns and, of course, haven't seen the message since, but I've changed Settings - System Administration - Startup and Shutdown (Bottom Right) - Session Management - On login to Start with an empty session anyway.