Kivy:在另一个类中更新标签文本的函数调用不起作用

from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.button import Button
from kivy.uix.boxlayout import BoxLayout
from kivy.lang import Builder
from kivy.event import EventDispatcher


class CustomEvent(EventDispatcher):
    def custom_event(self):
        self.dispatch('on_custom_event')

    def on_custom_event(self):
        pass


class MyLabel(Label):
    def __init__(self, **kwargs):
        super(MyLabel, self).__init__(**kwargs)

    def update_text(self, new_text):
        self.text = new_text


class MyBoxLayout(BoxLayout):
    def __init__(self, **kwargs):
        super(MyBoxLayout, self).__init__(**kwargs)

        self.label = MyLabel(text='Original Text')
        self.button = Button(text='Update Text')
        self.button.bind(on_release=self.update_label_text)
        self.add_widget(self.label)
        self.add_widget(self.button)

    def update_label_text(self, instance):
        self.label.update_text('New Text')


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


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