屏幕管理器每当创建多个屏幕时都无法正常工作

Question

我正在尝试使用多个窗口进行测验游戏。第一个窗口是定义游戏规则，第二个窗口将具有游戏及其问题。但是，每当我试图一次又一次地面对相同问题的程序时。 合适的说明

kv = Builder.load_file('StandAloneQuiz.kv')
   File "/home/shazam/.local/lib/python3.8/site-packages/kivy/lang/builder.py", line 305, in load_file
     return self.load_string(data, **kwargs)
   File "/home/shazam/.local/lib/python3.8/site-packages/kivy/lang/builder.py", line 407, in load_string
     self._apply_rule(
   File "/home/shazam/.local/lib/python3.8/site-packages/kivy/lang/builder.py", line 620, in _apply_rule
     cls = Factory_get(cname)
   File "/home/shazam/.local/lib/python3.8/site-packages/kivy/factory.py", line 147, in __getattr__
     raise FactoryException('Unknown class <%s>' % name)
 kivy.factory.FactoryException: Unknown class <IndexPage>

这是.kv文件的代码：

#:kivy 2.1.0

ScreenManager:
    IndexPage:
    Quizstart:

<IndexPage>:
    name:"Instruction"
    BoxLayout:
        orientation:'vertical'
        padding:10
        spacing:8
        Label:
            text:'Rules of the Game'
            font_size:25
        Label:
            text:'1. Time limit of the quiz is 10 min\n2. Only 1 option is correct choose wisely.\n3. Good luck.'
        Button:
            text:'Start'
            # id:startquiz
            on_release:
                app.root.current:'Quizwin'
                root.manager.transition.direction='right'
<Quizstart>:
    name:"QuizWin"
    BoxLayout:
        orientation:'vertical'
        padding:10
        spacing:8
        Label:
            text:'Questions will arrive shortly'
        Button:
            text:'goBack'
            on_release:
                app.root.current:'Instruction'
                root.manager.transition.direction='left'

这是Main .py文件的代码：

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.screenmanager import Screen,ScreenManager

kv = Builder.load_file('StandAloneQuiz.kv')

class IndexPage(Screen):
    pass

class Quizstart(Screen):
    pass

class ScreenManagerial(ScreenManager):
    pass

class StandAlone(App):
    def build(self):
        return kv

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