I was able to get rid of the Gtk Warning into Places List view.
The fix is not nice: inclusion of expected action, set into a private class (PageView)... :(
Note, it seems that to set action enlights a possible previous workaround!
I do not fully understand relations between GotoMap and MapsList.
Anyway, it is just warning around GUI.
PS: I have a Gtk Warning on Place View (Gramps 4.0.x)
(Gramps.py:2583): Gtk-WARNING **: MapsList: missing action MapsList