问AttributeError：'NoneType‘对象没有属性'setDelegate_’，如何修复这个问题？

EN

from kivymd.app import MDApp
from kivy.lang.builder import Builder
from kivy.uix.screenmanager import ScreenManager, Screen
from kivymd.uix.picker import MDDatePicker
from kivymd.uix.picker import MDThemePicker
from kivy.config import Config
from kivy.uix.boxlayout import BoxLayout
from kivymd.theming import ThemableBehavior
from kivymd.uix.list import MDList
from kivy.properties import ObjectProperty
from kivymd.uix.navigationdrawer import MDNavigationDrawer
from kivy.properties import StringProperty, ListProperty
from kivy.lang import Builder
from kivy.core.window import Window
from plyer import notification
import time
import plyer


Config.set('graphics', 'width', '1600')
Config.set('graphics', 'height', '1200')


KV = '''
<Box@BoxLayout>:
    bg: .65, .48, .35, 1 
       
             

BoxLayout:
    Rectangle:    
        size: 1600, 1200

    Box:
        bg: app.theme_cls.bg_light
    Box:
        bg: app.theme_cls.bg_normal
    Box:
        bg: app.theme_cls.bg_dark
    Box:
        bg: app.theme_cls.bg_darkest
            
'''
            
screen_helper = """
ScreenManager:
    MenuScreen:
    HomeScreen:
    BreakScreen:
    SettingsScreen
        
<MenuScreen>:
    name: 'menu'
    
  
        
    MDLabel:
        rectangle:
        background_color: .65, .48, .35, 1           
        size: 300, 700
        pos_hint: {'center_x':0.1, 'center_y':0.0}
        
                
    MDLabel:
        text: "Welcome to"
        font_style: 'H2'
        size:500, 500
        pos_hint: {'center_x':0.95,'center_y':0.84}
        
    MDLabel:
        text: "LEAF"
        font_style: 'H3'
        size:500, 500
        pos_hint: {'center_x':1.045,'center_y':0.7}
        
        
    MDFlatButton:
        text: 'Calendar'
        font_style: 'H6'
        pos_hint: {'center_x':0.091,'center_y':0.65}
        on_release: app.show_date_picker()
            
            
    MDFlatButton:
        text: 'Home'
        font_style: 'H6'
        pos_hint: {'center_x':0.076,'center_y':0.57}
        on_press: root.manager.current = 'home'
        
    MDFlatButton:
        text: 'Settings'
        font_style: 'H6'
        pos_hint: {'center_x':0.09,'center_y':0.49}
        on_press: root.manager.current = 'settings'
        
            
    MDRectangleFlatButton:
        text: 'Continue'
        font_style: 'H6'
        pos_hint: {'center_x':0.61,'center_y':0.345}
        on_press: root.manager.current = 'home'
   


    
<HomeScreen>:
    name: 'home'    
                
        
    MDToolbar:
        id: toolbar
        title: "Home
        pos_hint: {"top": 1}
        elevation: 5
        left_action_items: [["menu", lambda x: nav_drawer.set_state("open")]]
        
    Widget:

        MDNavigationDrawer:
            id: nav_drawer
    
            ContentNavigationDrawer:
                orientation: 'vertical'
                padding: "8dp"
                spacing: "8dp"
                

                MDFlatButton:
                    text: 'Return to menu'
                    font_style: 'Subtitle1'
                    on_release: root.manager.current = 'menu'

                MDFlatButton:
                    text: 'Open Calendar'
                    font_style: 'Subtitle1'
                    on_release: app.show_date_picker()
                    
                MDFlatButton:
                    text: 'Open Settings'
                    font_style: 'Subtitle1'
                    on_press: root.manager.current = 'settings'



    
                    
                    
                            
                
                
            
    
<BreakScreen>:
    name: 'break'
    
<SettingsScreen>:
    name: 'settings'
    
    
    MDFlatButton:
        text: 'Return to menu'
        font_style: 'H6'
        pos_hint: {'center_x':0.1,'center_y':0.05}
        on_press: root.manager.current = 'menu'
        
    MDFlatButton:
        text: 'Change Theme'
        font_style: 'H6'
        pos_hint: {'center_x':0.85,'center_y':0.95}
        on_release: app.show_theme_picker() 
        
    MDFloatingActionButton:
        icon: 'moon-waning-crescent'
        theme_text_color: "Custom"
        md_bg_color: 0, 0.039, 0.867, 0.557
        pos_hint: {'center_x':0.9,'center_y':0.3}
        on_press: self.theme_cls.theme_style = "Dark"  # "Light"
        
    MDFloatingActionButton:
        icon: 'MDFloatingActionButton'
        icon: 'lightbulb'
        pos_hint: {'center_x':0.9,'center_y':0.1}
        on_press: self.theme_cls.theme_style = "Light"  # "Dark"
        
        
"""  

class MenuScreen(Screen):
    pass


class HomeScreen(Screen):
    pass


class BreakScreen(Screen):
    pass

class SettingsScreen(Screen):
    pass

class ContentNavigationDrawer(BoxLayout):
    pass

class DrawerList(ThemableBehavior, MDList):
    pass


    
sm = ScreenManager()
sm.add_widget(MenuScreen(name='menu'))
sm.add_widget(HomeScreen(name='profile'))
sm.add_widget(BreakScreen(name='upload'))
sm.add_widget(SettingsScreen(name='settings'))



class LeafApp(MDApp):
    
    def show_date_picker(self):
        date_dialog = MDDatePicker()
        date_dialog.open()

    def show_theme_picker(self):
        theme_dialog = MDThemePicker()
        theme_dialog.open()



    def notifyMe(ttle, msg):
        notification.notify(
            title = ttle,
            message = msg,
            timeout = 10,
    )


    if __name__ == '__main__':
        while True:
            notifyMe("Hey You! take a break now !!", "You should follow the 20-20-20 rule to keep your eyes healthy")
            time.sleep(1200)
            



    def build(self):
        screen = Builder.load_string(screen_helper)
        return screen
    
LeafApp().run()
'''