the .kv file is not being attached on Ubuntu

I just started using Python KivyI have this two files

# main.pyfrom kivy.app import Appfrom kivy.uix.button import Buttonfrom kivy.uix.boxlayout import BoxLayoutclass Container(BoxLayout):    passclass MyApp(App):    def build(self):        return Container()if __name__ == '__main__':    MyApp().run()

and

# My.kv<Container>:    orientation: 'vertical'    padding: 50    spacing: 25    Button:        size: 100, 100        size_hint: None, None        text: 'Hello'    Button:        text: 'World'

everything works on Windows, but I need Linux to build it.

if I run the same files on Ubuntu 18.04, I will get a black screen. When working without .kv there is no such problem, all widgets are displayed.

What's the matter?