title: How to fix AttributeError: 'NoneType' object has no attribute 'theme_cls'
body: <p>I am trying the Kivymd but keep getting the error: AttributeError: 'NoneType' object has no attribute 'theme_cls'. </p>
  
  <p>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.</p>
  
  <pre><code>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:
  
  
  &lt;Screen&gt;:
  
      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()
  </code></pre>