I am reading a book about python, kv language. the first step in this book is to create simple layout that render a custom widget with its children. the problem is that when I run the program it shows blank screen. no error message

weather.py file

from kivy.app import App

class WeatherApp(App):
 pass
if __name__ == '__main__':
 WeatherApp().run()

weather.kv file

<AddLocationForm@BoxLayout>: 
 orientation: "vertical"
 BoxLayout:
 TextInput:
 Button:
  text: "Search"
 Button:
  text: "Current Location"
 ListView:
  item_strings: ["Palo Alto, MX", "Palo Alto, US"]