title: AttributeError: 'NoneType' object has no attribute 'current'
body: <pre><code>class LoginScreen(Screen):
      def __init__(self,**kwargs):
          super(LoginScreen, self).__init__(**kwargs)
          print self,self.parent.current
  
  class AppScreenManager(ScreenManager):
      pass
  
  #Base Class
  class AppBaseClass(App):
      def build(self):
          icon='app_icon'
          return Builder.load_file('appbase.kv')
  
  
  ________________________________________________________________________________________________
  
  AppScreenManager:
      transition: FadeTransition()
      LoginScreen:
  </code></pre>
  
  <p>Error:  AttributeError: 'NoneType' object has no attribute 'current'. Please help.</p>