This has happened to me a couple of times and I'm unsure the right way around it.
On both occasions I ended up with a onPC window open that I can't Quit. If I press the Windows Red X or Alt+F4 the command line says OK:Shutdown but nothing happens. If I press the Power button on the control bar the command line shows OK:Shutdown /NoAutoClose but again nothing happens. If I open the Task Manager and end the root process onPC quits and restarts in the same state. If I End the Display Process the the application briefly goes "Not Responding" then comes back as it was.
I tried ReloadUI, Shutdown and Restart but none of them allow me to successfully shutdown the console.
I was able to use ProcessExplorer from Sysinternals to Kill app_system.exe or restarting the computer also works.
Edit: I noticed once in this state, even if I'm able to close the application using ProcessExplorer If I open another instance I continue to run into the issue until I reboot the machine.