title: button inside MDNavigationDrawer is not changing screen on release
body: <p>In my MDNavigationDrawer I put a button from which I want to change to a new screen  'About Us' but on the click, it is not responding
  The button is inside root widget of ContentNavigationDrawer in .kv file
  and I want to switch screen in AboutUs class but it is not responding</p>
  
  <p>here's my class of About us in .py file</p>
  
  <pre><code>Class AboutUS(BoxLayout):
      pass
  </code></pre>
  
  <p>here's the code in .kv file</p>
  
  <pre><code>Screen:
      AboutUs:
  
      MainScreen:
  
      NavigationLayout:
  
          ScreenManager:
  
              Screen:
  
                  BoxLayout:
                      orientation: 'vertical'
  
                      MDToolbar:
                          title: "Story A Day"
                          anchor_title: 'left'
                          elevation: 10
                          left_action_items: 
                              [['menu',lambdax:nav_drawer.set_state()]]
                          mode: "free-center"
                          right_action_items: 
                              [["jellyfish", lambda x: print('HELLOFK')]]
  
                      Widget:
  
  
  
          MDNavigationDrawer:
              title: 'Story A Day'
              id: nav_drawer
              swipe_distance: 10
  
              ContentNavigationDrawer:
                  id: content_drawer
  
  &lt;ContentNavigationDrawer&gt;
      background_color: 2, 3, 4, 5
      orientation: "vertical"
      padding: "8dp"
      spacing: "8dp"
  
      AnchorLayout:
          anchor_x: "right"
          size_hint_y: None
          height: avatar.height
  
          Image:
              id: avatar
              size_hint: None, None
              size: "56dp", "56dp"
              source: "data/logo/kivy-icon-256.png"
  
          MDLabel:
              anchor_y: 'left'
              text: "[b]Story A Day[/b]"
              theme_text_color: "Custom"
              text_color: 0, 0, 0, 1
              markup: True
              font_style: "H4"
              size_hint_y: None
              height: self.texture_size[1]
  
      MDRaisedButton:
          md_bg_color: 0, 0, 0, 1
          text: 'You Can'
          on_release: root.hllo()
  
      MDRaisedButton:
          text: 'Click Me'
          on_release: print('Hell')
  
      MDLabel: 
          text: 'Story A Day'
          font_style: "Button"
          size_hint_y: None
          height: self.texture_size[1]
  
      MDExpansionPanelTwoLine:
          text: 'send us your idea'
          secondary_text: 'email: xxxxx@gmail.com'
  
          IconLeftWidget:
              icon: "email"
  
      OneLineAvatarListItem:
          text: "About Us"
          on_release:
              app.current = "abt"
  
          IconLeftWidget:
              icon: "information"
              on_press: print('a')
  
      ScrollView:
  
  &lt;AboutUs&gt;:
      id: manager
      name: 'abt'
  
      Label:
          text: "About Us"
  </code></pre>