如何在KIvy中使用弹出窗口小部件(按钮)从类中访问小部件/id

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button
from kivy.uix.popup import Popup
from kivy.uix.label import Label

class MyBoxLayout(BoxLayout):
    def __init__(self, **kwargs):
        super(MyBoxLayout, self).__init__(**kwargs)
        
        # 创建弹出窗口按钮
        popup_button = Button(text='弹出窗口', on_press=self.show_popup)
        
        # 添加其他小部件
        label = Label(text='这是一个标签')
        
        # 将按钮和其他小部件添加到布局中
        self.add_widget(popup_button)
        self.add_widget(label)
    
    def show_popup(self, instance):
        # 创建弹出窗口内容
        popup_content = Label(text='这是一个弹出窗口')
        
        # 创建弹出窗口
        popup = Popup(title='弹出窗口', content=popup_content, size_hint=(None, None), size=(400, 400))
        
        # 打开弹出窗口
        popup.open()

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

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