Don't set favorite is menu screen is not set

issue3387
review1130002
(transplanted from cddf1ac68ad1a4e82bd4e920df1e674cef5f61a0)

tryton/gui/main.py

@@ -811,6 +811,9 @@ class Main(object):
         return _MAIN[0]
 
     def favorite_set(self):
+        if not self.menu_screen:
+            return
+
         def _action_favorite(widget, id_):
             Action.exec_keyword('tree_open', {
                 'model': self.menu_screen.model_name,