Fix switch view when focus on many2one and record is not saved.

tryton/gui/window/view_form/view/form_gtk/many2one.py

@@ -224,6 +224,8 @@ class Many2One(WidgetInterface):
     def sig_activate(self, widget, event=None):
         if not self.focus_out:
             return
+        if not self._view.modelfield:
+            return
         self.changed = False
         value = self._view.modelfield.get(self._view.model)