如何修复AttributeError：'NoneType'对象没有属性'theme_cls'

Question

I am trying the Kivymd but keep getting the error: AttributeError: 'NoneType' object has no attribute 'theme_cls'.

I added the theme_cls = ThemeManager() in the App class but am still getting the error. Please help as there are no more solutions online.

from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen, FadeTransition, WipeTransition

from kivy.app import App
from kivy.uix.button import Button
from kivymd.navigationdrawer import MDNavigationDrawer
from kivymd.theming import ThemeManager

class BeginScreen(Screen):
    pass

class MyScreenManager(ScreenManager):
    pass

Login = Builder.load_string('''
#: import Toolbar kivymd.toolbar.Toolbar


MyScreenManager:
    Screen:


<Screen>:

    Toolbar:
        title: "Simple toolbar"
        pos_hint: {'center_x': 0.5, 'center_y': 0.75}
        md_bg_color: get_color_from_hex(colors['Teal']['500'])
        background_palette: 'Teal'
        background_hue: '500'
    Toolbar:
        title: "Toolbar with right buttons"
        pos_hint: {'center_x': 0.5, 'center_y': 0.5}
        md_bg_color: get_color_from_hex(colors['Amber']['700'])
        background_palette: 'Amber'
        background_hue: '700'
        right_action_items: [['content-copy', lambda x: None]]
    Toolbar:
        title: "Toolbar with left and right buttons"
        pos_hint: {'center_x': 0.5, 'center_y': 0.25}
        md_bg_color: get_color_from_hex(colors['DeepPurple']['A400'])
        background_palette: 'DeepPurple'
        background_hue: 'A400'
        left_action_items: [['arrow-left', lambda x: None]]
        right_action_items: [['lock', lambda x: None], \
            ['camera', lambda x: None], \
            ['play', lambda x: None]]


''')

class MyApp(App):
    theme_cls = ThemeManager()

    def build(self):
        return Login

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

Answer 1

Root Cause

When Kivy process the kv file / string, it instantiated the root rule , MyScreenManager: before the instantiation of theme_cls . When MyScreenManager: is instantiated, it instantiates its child, Screen: . When Screen: is instantiated, it instantiates its children, Toolbar: which references theme_cls . But theme_cls is not yet instantiated. Therefore, it threw the following error,

  self._shadow = App.get_running_app().theme_cls.quad_shadow 

AttributeError: 'NoneType' object has no attribute 'theme_cls'

Solution

kv string

• Replace root rule, MyScreenManager: with class rule, <MyScreenManager>:

Snippets

Login = Builder.load_string('''
#:import Toolbar kivymd.toolbar.Toolbar


<MyScreenManager>:
    Screen:

Python script

• Replace return Login with return MyScreenManager()

Snippets

class MyApp(App):
    theme_cls = ThemeManager()

    def build(self):
        return MyScreenManager()

Output