*model* a `Model` instance, or None.

def set_model(self, model):
        """
        *model* a `Model` instance, or None.
        """
        if self.model:
            self.model.unregister_observer(self)
        self.model = model
        if self.model:
            self.model.register_observer(self)
            self.model_notification()
        else:
            self.set_widget(None)
        self.widget.set_sensitive(bool(self.model))