Ubuntu 18.10 and 18.04 use the (K)StatusNotifierItem/AppIndicator Support extension for Gnome Shell by default. There's a bug that can occur on both Ubuntu 18.04 and 18.10 for some users, which results in losing the indicator icons from the top bar.
I'm not sure what exactly triggers the missing indicator icons bug, but reloading Gnome Shell (
Alt + F2 and then typing r) gets the indicator icons to show up. When running a new application that uses an indicator icon, it doesn't appear until Gnome Shell is restarted again.What's more, if you lock the screen and then unlock it, the icon are missing again until Gnome Shell is reloaded.