System configuration settings button disappeared in Ubuntu 19.04
The problem was gnome-settings-daemon wasn't installed. When I ran:
$ gnome-control-center
I got:
Command 'gnome-control-center' not found, but can be installed with:
sudo apt install gnome-control-center
So I installed that using:
sudo apt install gnome-control-center
The configuration button now appears in the application and can be configured. It also appeared on the side of the power off button that was not then after restarting.