Kivy Multiple Screens

#encoding: utf-8
from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.label import Label
from kivy.uix.button import Button
from kivy.uix.screenmanager import ScreenManager, Screen

class MainScreen(Screen):
    pass

class AboutScreen(Screen):
    pass

class PlanLekcji(App):
    def build(self):
        sm = ScreenManager()
        sm.add_widget(MainScreen(name='main'))
        sm.add_widget(AboutScreen(name='about'))
        return sm


if __name__ == "__main__":
    PlanLekcji().run()