Install the package gtk3-mushrooms coming from AUR.
Or get rid of the system panel icons altogether.
The reasoning is that we can reach those applications if we want to via shortcuts or terminal.
We will show you the last solution in this video.
You can get rid of the system icons with this line of code
#system icons tray_output none
Anno 18/12/2018 this is no longer an issue and icons are displayed correctly with gtk3 3.24.2+7+g43aeb52b73-1.