Missing system tray icons Ubuntu Gnome 18.04

You may also simply try to relaunch gnome shell and see if it works better:

(Alt+F2) gnome-shell --replace

Or the short and easy way

(Alt+F2) r (Enter)

Yes, just the letter "r".

I had it a few times that after an 18.04 update the icons were missing. In the past this fixed it:

sudo cpan -i Gtk2::AppIndicator

But this time this didn't worked. So removing Unity fixed it for me:

sudo apt purge unity-session unity

For the readers: only remove Unity when you are using the Gnome desktop. ;-)

1. Log in to Synaptic.

2. Type in search: libappindicator

3. Mark packages for installation:

   libappindicator0.1-cil
   libappindicator3-1
   libappindicator3-dev
   

4. Other libappindicator packages - if they are installed - mark for removal.