Cannot delete "On My Mac" mailbox in Mail v16 in MacOS Monterey

I'm just trying to delete a Mail (v16.0) mailbox (which I created long ago) saved in the "On My Mac" section and nothing works. I am don't want to delete a Mail account, just a specific mailbox from which I've already deleted the mail contents:

• I right-click on the mailbox item in the "On My Mac" section of the lefthand listing, select Delete Mailbox... option, Click delete, and the Mail app crashes! Mailbox still there when I re-open mail.

• Via Finder, I went into Library/Mail/V9/ found the folder with the mailbox I want to delete, send it to the Trash, empty trash. Reboot Mac, open Mail -- Mailbox is still there.

Any suggestions would be appreciated!