How do I cancel a stale "pending" drive mount with error "An operation is already pending"

The An operation is already pending error is very generic and many things can cause this. (other answers don't relate to my problem). I have a Luks encrypted volume that I clicked to open, a password prompt appeared, and I waited too long to enter the password. (It normally works just fine.) Now, whenever I try to open it, I just get that error instead of the password prompt. I'm sure rebooting the computer would fix it, but I don't want to do that... I'm sure it's just some sort of mount lock that needs to be manually released. How do I release the lock manually so I can try again? The OS is Mate on Mint 17.3.