我正在尝试在 kivy 中切换屏幕，但我已经坚持了一段时间，所以我不知道发生了什么。 文本正在打印，但屏幕仍然没有变化。 如果有人可以帮助我，这是python代码：

from kivymd.app import MDApp
from kivy.lang import Builder
from kivy.core.window import Window
from kivy.uix.boxlayout import BoxLayout
from kivymd.theming import ThemableBehavior
from kivymd.uix.list import MDList
from kivy.uix.screenmanager import Screenmanager,Screen,SlideTransition
from kivymd.uix.button import MDFloatingBottomButton



Window.size = (300,500)

class MainScreen(Screen):
    def switch_button(self):
        print("switching")
        Screenmanager.current = "FileScreen"  
        

class FileScreen(Screen):
    pass


class app(MDApp):
    data = {
        'Create': 'file-document','Open': 'folder-open'
        
    }
    def btn(self,instance):
        print('callback')
        icon = instance.icon
        # if you want check button,use
        if isinstance(instance,MDFloatingBottomButton):

            if icon == 'file-document':
                print('Create')
                MainScreen.switch_button(self)
                
                
                
            elif icon == 'folder-open':
                print('Open')

    class ContentNavigationDrawer(BoxLayout):
        pass

    class DrawerList(ThemableBehavior,MDList):
        pass

        sm = Screenmanager()
        sm.add_widget(MainScreen(name='MainScreen'))
        sm.add_widget(FileScreen(name='FileScreen'))
        
    def build(self):
        self.theme_cls.primary_palette = "Red"
        #self.theme_cls.accent_palette = "Teal"
        screen = Builder.load_file("main.kv")
        return screen
        
        

    

app().run()

这是.KV代码：

Screenmanager:
    MainScreen:
    FileScreen:
    
<MainScreen>:
    name: "MainScreen"
    MDFloatingactionButtonSpeedDial:
        bg_hint_color: app.theme_cls.primary_light
        data: app.data
        root_button_anim: True
        callback: app.btn

<FileScreen>:
    name: "FileScreen"
    MDBoxLayout:
        MDLabel:
            text: "test"

我希望社区能尽快帮助我，因为如果我已经坚持了一段时间。