带有.kv文件的Kivy应用程序无法显示

Question

我正在尝试修改此示例：https://github.com/inclement/kivycrashcourse/blob/master/video14-using_a_screenmanager/after.py以使其与.kv文件一起使用。这是我的myscreenmanager.py文件：

from kivy.app import App
from kivy.base import runTouchApp
from kivy.lang import Builder
from kivy.properties import ListProperty
from kivy.uix.boxlayout import BoxLayout

from kivy.uix.screenmanager import ScreenManager, Screen, FadeTransition

import time
import random

class FirstScreen(Screen):
    pass

class SecondScreen(Screen):
    pass

class ColourScreen(Screen):
    colour = ListProperty([1., 0., 0., 1.])

class MyScreenManager(ScreenManager):
    def new_colour_screen(self):
        name = str(time.time())
        s = ColourScreen(name=name,
                         colour=[random.random() for _ in range(3)] + [1])
        self.add_widget(s)
        self.current = name

class MyScreenManagerApp(App):
    def build(self):
        return MyScreenManager()

if __name__ == "__main__":
    MyScreenManagerApp().run()

这是我的myscreenmanager.kv文件：

#:import FadeTransition kivy.uix.screenmanager.FadeTransition
MyScreenManager:
    transition: FadeTransition()
    FirstScreen:
    SecondScreen:

<FirstScreen>:
    name: 'first'
    BoxLayout:
        orientation: 'vertical'
        Label:
            text: 'first screen!'
            font_size: 30
        BoxLayout:
            Button:
                text: 'goto second screen'
                font_size: 30
                on_release: app.root.current = 'second'
            Button:
                text: 'get random colour screen'
                font_size: 30
                on_release: app.root.new_colour_screen()

<SecondScreen>:
    name: 'second'
    BoxLayout:
        orientation: 'vertical'
        Label:
            text: 'second screen!'
            font_size: 30
        BoxLayout:
            Button:
                text: 'goto first screen'
                font_size: 30
                on_release: app.root.current = 'first'
            Button:
                text: 'get random colour screen'
                font_size: 30
                on_release: app.root.new_colour_screen()

<ColourScreen>:
    BoxLayout:
        orientation: 'vertical'
        Label:
            text: 'colour {:.2},{:.2},{:.2} screen'.format(*root.colour[:3])
            font_size: 30
        Widget:
            canvas:
                Color:
                    rgba: root.colour
                Ellipse:
                    pos: self.pos
                    size: self.size
        BoxLayout:
            Button:
                text: 'goto first screen'
                font_size: 30
                on_release: app.root.current = 'first'
            Button:
                text: 'get random colour screen'
                font_size: 30
                on_release: app.root.new_colour_screen()

运行应用程序后，屏幕上不显示任何内容。控制台没有错误。切换回Builder.load_string会按预期显示应用程序。