Update:
The Mageia bug that I posted does not appear to have been sent upstream, so I posted the problem on the KDE page with the WidgetListCrash program.
https://bugs.kde.org/show_bug.cgi?id=399989
They seem to have tracked it down pretty well and also think it is a Qt fix. I have no idea where or how to reopen the Qt bug report or which bug that refers to. I am similarly in the dark with respect to the KDE plugin.
Banjo
(_)=='=~