title: Screen Manager in kivy with kv file
body: <p>this is the first question I am posting so sorry if the question is not complete.</p>
  
  <p>I tried the example on kivy's official page for Screen Manager and it works, but if I try using a kv file instead of the Builder, I get a black page.</p>
  
  <p>Test.py</p>
  
  <pre><code>from kivy.app import App
  from kivy.lang import Builder
  from kivy.uix.screenmanager import ScreenManager, Screen
  
  class MenuScreen(Screen):
      pass
  
  class SettingsScreen(Screen):
      pass
  
  # Create the screen manager
  sm = ScreenManager()
  sm.add_widget(MenuScreen(name='menu'))
  sm.add_widget(SettingsScreen(name='settings'))
  
  class TestApp(App):
  
      def build(self):
          return sm
  
  if __name__ == '__main__':
      TestApp().run()
  </code></pre>
  
  <p>Test.kv</p>
  
  <pre><code>&lt;MenuScreen&gt;:
      BoxLayout:
          Button:
              text: 'Goto settings'
              on_press: root.manager.current = 'settings'
          Button:
              text: 'Quit'
  
  &lt;SettingsScreen&gt;:
      BoxLayout:
          Button:
              text: 'My settings button'
          Button:
              text: 'Back to menu'
              on_press: root.manager.current = 'menu'
  </code></pre>