I had to kill Anki’s process to “quit” it. This error was presented on each UI Quit attempt. I was later able to reproduce by adding a deck and deleting it in the same session.
FWIW I’ll mention that I have only three add-ons enabled.
Error
An error occurred. Please start Anki while holding down the shift key, which will temporarily disable the add-ons you have installed.
If the issue only occurs when add-ons are enabled, please use the Tools > Add-ons menu item to disable some add-ons and restart Anki, repeating until you discover the add-on that is causing the problem.
When you’ve discovered the add-on that is causing the problem, please report the issue on the add-on support site.
Debug info:
Anki 2.1.42 (8eebfe18) Python 3.8.6 Qt 5.14.2 PyQt 5.14.2
Platform: Mac 10.15.7
Flags: frz=True ao=True sv=2
Add-ons, last update check: 2021-03-30 19:31:49
Caught exception:
Traceback (most recent call last):
File “aqt/main.py”, line 1018, in closeEvent
File “aqt/main.py”, line 451, in unloadProfileAndExit
File “aqt/main.py”, line 428, in unloadProfile
File “aqt/main.py”, line 537, in unloadCollection
File “aqt/main.py”, line 808, in closeAllWindows
File “aqt/init.py”, line 133, in closeAll
File “aqt/browser.py”, line 586, in closeWithCallback
File “aqt/editor.py”, line 533, in saveNow
File “aqt/editor.py”, line 652, in saveTags
File “anki/notes.py”, line 70, in flush
File “anki/_backend/generated.py”, line 430, in update_note
File “anki/_backend/init.py”, line 106, in _run_command
anki.errors.NotFoundError