如何在kivy中刷新弹出窗口的背景图像？

from kivy.app import App
from kivy.uix.popup import Popup
from kivy.uix.image import Image
from kivy.uix.boxlayout import BoxLayout
from kivy.clock import Clock

class CustomPopup(Popup):
    def __init__(self, **kwargs):
        super(CustomPopup, self).__init__(**kwargs)
        self.background_image = Image(source='background.jpg')

    def refresh_background(self):
        self.background_image.reload()

class MainLayout(BoxLayout):
    def __init__(self, **kwargs):
        super(MainLayout, self).__init__(**kwargs)
        self.orientation = 'vertical'
        self.padding = 10

        self.popup_button = Button(text='Open Popup')
        self.popup_button.bind(on_release=self.open_popup)
        self.add_widget(self.popup_button)

    def open_popup(self, instance):
        popup = CustomPopup()
        popup.open()
        Clock.schedule_once(lambda dt: popup.refresh_background())

class MyApp(App):
    def build(self):
        return MainLayout()

if __name__ == '__main__':
    MyApp().run()