I started seeing this after updating Fedora 29 (KDE) which installed kf5-*-5.59.
There are several postings on StackOverflow (unix.stackexchange.com, superuser.com) about this issue and suggestions on how to suppress the notification. There's a Fedora bug report: https://bugzilla.redhat.com/show_bug.cgi?id=1746465 and a KDE bug report: https://bugs.kde.org/show_bug.cgi?id=411441
This message happens whenever I close Dolphin, but also at random other times.
Apparently, downgrading to kf5-kio-5.58 reverts the code which caused the error. Unfortunately, this is not in the F29 repo so it isn't possible to use dnf to downgrade. Can someone update the repo with the old version?