grabHelper is passing a boolean argument to onUngrab() callback function
and since commit 1acbdcc9b3b0 we'd end up adding it to to the callback list or
we'd try to invoke it:
(gnome-shell:3490851): Gjs-CRITICAL **: 17:19:20.460: JS ERROR:
TypeError: func is not a function
onComplete/<@/media/M2/GNOME/gnome-shell/js/ui/appDisplay.js:2407:56
onComplete@/media/M2/GNOME/gnome-shell/js/ui/appDisplay.js:2407:40
_makeEaseCallback/<@/media/M2/GNOME/gnome-shell/js/ui/environment.js:85:13
_easeActor/<@/media/M2/GNOME/gnome-shell/js/ui/environment.js:170:64
Use an arrow function so that we can control the parameters we pass
to popdown.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1635>