How to switch off UR22 mkII to avoid error message on Mac

I have just bought this audio interface. It is connected to my Mac desktop computer (Mojave 10.14). I do not know how to switch the interface off so I can turn my computer off safely. When I switch the computer off I get an error message on restarting caused by the audio interface. However there seems to be no on/off switch on the device, nor does it appear on the computer as added hardware which can be ejected. Do I just pull the USB cable out, or what?

(I can’t believe I’m even having to ask this :unamused: ).