After some of the last Plasma updates the Favorites in Classical Applications Menu (Kicker) became empty. It happened near a week ago in the same day in openSUSE Leap 42.1 with unstable developers Plasma repos and in Neon developers unstable branch, and soon after that also in KaOS with "kde-next" repo. It is still possible to add some application to Favorites, but after re-login or restarting Plasma the Favorites are empty again. I had created the bug report, please, confirm if you have the same problem:

https://bugs.kde.org/show_bug.cgi?id=374156