Unable to get a Window, abort. in kivy python

Question

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button
from kivy.uix.widget import Widget


class BoxLayoutExample(BoxLayout):
     def __init__(self, **kwargs):
         super().__init__(**kwargs)
         b1 = Button(text='A')
         b2 = Button(text='B')
         self.add_widget(b1)
         self.add_widget(b2)


class MainWidget(Widget):
    pass

class TheLabApp(App):
    pass

TheLabApp().run()

in this case, I couldn't get the new app window, an error. and actually, I didn't understand it