Update: Please see EricS's answer first as you may be able to avoid losing your KDE config.

I ended up removing random config files with the name plasma in them. The one that worked was:

cd ~/.config
mv plasma-org.kde.plasma.desktop-appletsrc \
   plasma-org.kde.plasma.desktop-appletsrc.bak

Then restart plasmashell:

kbuildsycoca5 && kquitapp plasmashell && kstart plasmashell

Update: Please see EricS's answer first as you may be able to avoid losing your KDE config.

I ended up removing random config files with the name plasma in them. The one that worked was:

cd ~/.config
mv plasma-org.kde.plasma.desktop-appletsrc \
   plasma-org.kde.plasma.desktop-appletsrc.bak

Then restart plasmashell:

kbuildsycoca5 && kquitapp plasmashell && kstart plasmashell

I ended up removing random config files with the name plasma in them. The one that worked was:

cd ~/.config
mv plasma-org.kde.plasma.desktop-appletsrc \
   plasma-org.kde.plasma.desktop-appletsrc.bak

Then restart plasmashell:

kbuildsycoca5 && kquitapp plasmashell && kstart plasmashell

Update: Please see EricS's answer first as you may be able to avoid losing your KDE config.

I ended up removing random config files with the name plasma in them. The one that worked was:

cd ~/.config
mv plasma-org.kde.plasma.desktop-appletsrc \
   plasma-org.kde.plasma.desktop-appletsrc.bak

Then restart plasmashell:

kbuildsycoca5 && kquitapp plasmashell && kstart plasmashell