Can't shut down MacPro, or logout because safari won't quit

I'm on a MacPro6,1 running MacOS 10.14.5. If I attempt to either logout, or shutdown the system, I get a popup that says

"Safari" interrupted logout (or restart). To continue logging out (restarting) quit "Safari".

Attempting to perform any action on the safari window results in a "donk" noise, but no action. I can't minimize, maximize, or resize the window, or use the red x to close it. And because safari won't close, I can't logout, or gracefully shutdown/restart.