title: Trouble with Kivy ScreenManager
body: <p>Lately I have some problems with Kivy's Screenmanager. I can't use the <code>switch_to</code> method of the manager when accessing it from a <code>Screen</code> class. In a <code>Screen</code> class: <code>self.manager.switch_to</code> gives the error: <code>AttributeError: 'NoneType' object has no attribute 'switch_to'</code>. I'm a bit desperate. Below simple app that produces the error. </p>
  
  <pre><code>import kivy
  kivy.require("1.9.1")
  
  from kivy.app import App
  from kivy.uix.label import Label
  from kivy.uix.screenmanager import ScreenManager, Screen
  
  screen1 = True
  
  
  class Screen1(Screen):
      def __init__(self, **kw):
          super(Screen1, self).__init__(**kw)
          self.add_widget(Label(text="Screen1"))
  
  
  class Screen2(Screen):
      def __init__(self, **kw):
          super(Screen2, self).__init__(**kw)
          self.add_widget(Label(text="Screen2"))
  
  
  class BlackMenu(Screen):
      def __init__(self, **kw):
          super(BlackMenu, self).__init__(**kw)
          if screen1:
              self.manager.switch_to(Screen1())
  
  
  class MyApp(App):
      def build(self):
          mymanager = ScreenManager()
          mymanager.switch_to(BlackMenu(name="black"))
          return mymanager
  
  
  if __name__ == "__main__":
      MyApp().run()
  </code></pre>