I just had the same thing happen with Dorico v2.2. First time. Dorico crashed when I was trying to edit a bit of text (the name of the staff appears on EVERY SINGLE LINE, its an xml import, and I cannot for the life of me figure out HOW to get rid of it, or to even select it. Frustrating). Then after Dorico crashed, Halion stayed up on the screen, and I cannot force quit it. It does not appear in the “Force Quit” window. One of the few times I’ve ever had to restart the machine. One thing I just installed was “Loopback” by Rogue Amoeba (great company), so that is the only change since I last used Dorico.
The audio engine of Dorico (and within there HALion Sonic) is running as a separate background process and therefore does not appear in the force-quit dialog. So Dorico crashed and the audio engine did continue running. To get rid of it, you need to invoke e.g. the Activity Monitor and look for a process called VSTAudioEngine and kill it from there.
Could you choose in Dorico from the main menu Help > Create Diagnostic Report? That will create a zip file on your desktop, please send to ‘u dot stoermer at steinberg dot de’. From the log information we might be able to see what caused Dorico crashing in first place.