Не отображаются виджеты, только чёрный экран

from kivy.app import App
from kivy.config import Config
 
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.gridlayout import GridLayout
from kivy.uix.button import Button
from kivy.uix.textinput import TextInput
 
Config.set("graphics", "resizable", "0")
Config.set("graphics", "width", "300")
Config.set("graphics", "height", "300")
 
saveInput = ""
 
class CalculatorApp(App):
 
    def calculate(self, symbol):
        global saveInput
        if symbol.text is '<':
            saveInput = self.result.text = ""
        elif symbol.text is not '=':
            self.result.text += symbol.text
            saveInput += symbol.text
        else:
            try: saveInput = self.result.text = str(eval(saveInput))
            except: saveInput = self.result.text = ""
        
    def build(self):
        root = BoxLayout(orientation = "vertical", padding = 5)
 
        self.result = TextInput(
            text = "", readonly = True, font_size = 25, 
            size_hint = [1, .75], background_color = [1, 1, 1, .8])
        root.add_widget(self.result)
 
        allButtons = GridLayout(cols = 5)
 
        allButtons.add_widget(Button(text = '7', on_press = self.calculate))
        allButtons.add_widget(Button(text = '8', on_press = self.calculate))
        allButtons.add_widget(Button(text = '9', on_press = self.calculate))
        allButtons.add_widget(Button(text = "+", on_press = self.calculate))
        allButtons.add_widget(Button(text = "<", on_press = self.calculate))
 
        allButtons.add_widget(Button(text = '4', on_press = self.calculate))
        allButtons.add_widget(Button(text = '5', on_press = self.calculate))
        allButtons.add_widget(Button(text = '6', on_press = self.calculate))
        allButtons.add_widget(Button(text = "-", on_press = self.calculate))
        allButtons.add_widget(Button(text = "(", on_press = self.calculate))
 
        allButtons.add_widget(Button(text = '1', on_press = self.calculate))
        allButtons.add_widget(Button(text = '2', on_press = self.calculate))
        allButtons.add_widget(Button(text = '3', on_press = self.calculate))
        allButtons.add_widget(Button(text = "*", on_press = self.calculate))
        allButtons.add_widget(Button(text = ")", on_press = self.calculate))
 
        allButtons.add_widget(Button(text = '0', on_press = self.calculate))
        allButtons.add_widget(Button(text = ".", on_press = self.calculate))
        allButtons.add_widget(Button(text = "=", on_press = self.calculate))
        allButtons.add_widget(Button(text = "/", on_press = self.calculate))
        allButtons.add_widget(Button(text = "%", on_press = self.calculate))
 
        root.add_widget(allButtons)
        return root
 
if __name__ == "__main__":
    CalculatorApp().run()