在Kivy中，如何在具有多个屏幕的应用程序中的特定屏幕上添加CircularProgressBar？

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.progressbar import ProgressBar
from kivy.uix.boxlayout import BoxLayout

class MyScreen(Screen):
    def __init__(self, **kwargs):
        super(MyScreen, self).__init__(**kwargs)
        
        # 创建一个BoxLayout作为屏幕的根布局
        layout = BoxLayout(orientation='vertical')
        
        # 创建一个CircularProgressBar并添加到BoxLayout中
        progress_bar = ProgressBar(max=100)
        layout.add_widget(progress_bar)
        
        # 将BoxLayout设置为屏幕的内容
        self.add_widget(layout)

class MyApp(App):
    def build(self):
        # 创建一个ScreenManager
        screen_manager = ScreenManager()
        
        # 创建一个MyScreen并添加到ScreenManager中
        screen = MyScreen(name='screen1')
        screen_manager.add_widget(screen)
        
        return screen_manager