I am working on an application with ScreenManager and getting AttributeError: 'NoneType' object has no attribute 'current' . Please help me to solve

shift.py `from kivy.app import App from kivy.uix.boxlayout import BoxLayout from kivy.lang import Builder from kivy.uix.screenmanager import ScreenManager ,Screen from kivy.uix.videoplayer import VideoPlayer Builder.load_file('shift.kv.py')

 class ConnectPage(Screen):
     pass
 class InfoPage(Screen):
     pass

 class Mobile(ScreenManager):
     pass

 class ShiftApp(App):
     def build(self):
         self.screen_manager=ScreenManager()

    
         self.connect_page = ConnectPage()
         screen = Screen(name='Connect')
         screen.add_widget(self.connect_page)
         self.screen_manager.add_widget(screen)

         self.info_page=InfoPage()
         screen=Screen(name='Info')
         screen.add_widget(self.info_page)
         self.screen_manager.add_widget(screen)
    
         return self.screen_manager
    

 if __name__=="__main__":
     ShiftApp().run()  `

 

shift.kv

`<ConnectPage>:
     Button:
         size_hint:.1 , .1
         pos_hint:{"bottom":1}
         text:"Welcome"
         on_release:
             root.manager.current='Info'
 

 <InfoPage>:
     Button:
         text:"Go Back!"
         on_release:
             root.manager.current='Connect'   `


 

Error

**File "C:\Users\paras\AppData\Local\Programs\Python\Python37\shift.kv.py", line 7, in root.manager.current='Info' AttributeError: 'NoneType' object has no attribute 'current' **