Where is magit-quit-session defined? It doesn't seem like a command
with that name has ever been in any Magit source files. Anyway, it
should update its call for the current signature of
magit-mode-quit-window, which takes one required argument, or perhaps
use magit-mode-bury-buffer instead.
Here are some of the relevant commits in case you want to look into it
further.
d4aba23 (magit-mode-quit-window: new function, 2015-08-20)
a44ed1b (rename magit-mode-{quit-window => bury-buffer}, 2015-01-23)
aa74eb9 (rename function magit-{ => mode}-quit-window, 2013-10-31)
--
Kyle