a #GActionGroup, or null
Deprecated: Use the #GActionMap interface instead. Never ever mix use of this API with use of #GActionMap on the same application or things will go very badly wrong. This function is known to introduce buggy behaviour (ie: signals not emitted on changes to the action group), so you should really use #GActionMap instead.
This used to be how actions were associated with a #GApplication. Now there is #GActionMap for that.