title: I can't put a list of expansion panels from on_start to a screen
body: <pre><code>from kivymd.app import MDApp
  from kivy.lang import Builder
  from kivy.uix.screenmanager import Screen, ScreenManager
  from kivymd.uix.expansionpanel import MDExpansionPanel, MDExpansionPanelOneLine
  from kivymd.uix.label import MDLabel
  
  gui = &quot;&quot;&quot;
  Screen:
      nav_drawer: nav_drawer
      MDToolbar:
          title: 'Welcome'
          pos_hint: {'top': 1}
          left_action_items: [['menu', lambda x: nav_drawer.set_state('toggle')]]
          
      MDNavigationLayout:
          id: nav_layout
          ScreenManager:
              id: screen_manager
              WelcomeScreen: 
          MDNavigationDrawer:
              id: nav_drawer
  &lt;WelcomeScreen&gt;:
      name: 'welcome'
      id: welcome
      
      ScrollView:
          GridLayout:
              cols: 1
              id: main
          
  &quot;&quot;&quot;
  md_list = [('List', 'The class MDList in combination with a BaseListItem like OneLineListItem will create a list that '
              'expands as items are added to it, working nicely with Kivy’s ScrollView.'),
             ('Banner', 'A banner displays a prominent message and related optional actions.')]
  
  
  class WelcomeScreen(Screen):
      pass
  
  
  sm = ScreenManager()
  sm.add_widget(WelcomeScreen(name='welcome'))
  
  
  class MyProject(MDApp):
      def build(self):
          screen = Builder.load_string(gui)
          return screen
  
      def on_start(self):
          for i in md_list:
              self.manager.get_screen('welcome').ids.main.add_widget(
                  MDExpansionPanel(
                      icon='calendar-text-outline',
                      content=MDLabel(text=i[1]),
                      panel_cls=MDExpansionPanelOneLine(
                          text=i[0])))
  
  
  MyProject().run()
  </code></pre>
  <p>This gave me:</p>
  <pre><code> File &quot;C:\Users\Hermi\PycharmProjects\pythonProject7\main.py&quot;, line 52, in on_start
       self.manager.get_screen('welcome').ids.main.add_widget(
   AttributeError: 'MyProject' object has no attribute 'manager'
  
  I have tried self.root.ids.main.add_widget, and got: 
  File &quot;C:\Users\Hermi\PycharmProjects\pythonProject7\main.py&quot;, line 52, in on_start
       self.root.ids.main.add_widget(
     File &quot;kivy\properties.pyx&quot;, line 864, in kivy.properties.ObservableDict.__getattr__
   AttributeError: 'super' object has no attribute '__getattr__'
  </code></pre>
  <p>I really don't understand how to navigate the widget tree. I've been looking all over.
  How can I place the expansion panels on the <code>WelcomeScreen</code>?</p>