[gnome-41] keyboard: Avoid segfault due to no "data" for `custom_shortcut_add_row`

Same as !1096 (merged) but against gnome-41. The issue seems to no longer occur on master.

Ultimately it would be good to get rid of the use of g_object_set_data/g_object_get_data here, but this works for now.

Fixes #1373 (closed).