After installing Kubuntu 18.10 many programs lack system tray icons: uGet, Clementine, VLC, Megasync client normally should have such icons in systray.
I have found this solution:
sudo apt remove sni-qt
Like the author of the linked post, I only had
sni-qt on my system. The other packages mentioned there were not installed (
After a while the above solution stopped working for uGet. It works for Megasync client though.
I notice that if I restart Plasma with all those programs opened, their icons appear in system tray. To restart Plasma I use a script:
#! /bin/sh -e killall plasmashell plasmashell &