a gtk.ActionGroup, 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.
2.28
This used to be how actions were associated with a gtk.Application Now there is GActionMap for that.