Моментально закрывается окно приложения kivy

from kivy.app import App
from kivy.uix.button import Button
from kivy.uix.widget import Widget
from kivy.uix.gridlayout import GridLayout
from kivy.uix.label import Label
from kivy.uix.boxlayout import BoxLayout
from kivy.config import Config

Config.set('graphics', 'resizable', 0)
Config.set('graphics', 'width', 400)
Config.set('graphics', 'height', 500)

class CalculatorApp(App):
    def update_label(self):
        self.lbl.text = self.formula

    def add_number(self, instance):
        if (self.formula == "0"):
            self.formula = ""

        self.formula += str(instance.text)
        self.update_label()

    def add_operation(self, instance):
        if (str(instance.text).lower() == "x"):
            self.formula += "*"
        else:
            self.formula += str(instance.text)

        self.update_label()

    def delete_number(self, instance):
        if (str(instance.text) == "C"):
            self.lbl.text = "0"
            self.formula = "0"

    def calc_result(self, instance):
        self.lbl.text = str(eval(self.lbl.text))
        self.formula = "0"

    def build(self):
        self.formula = "0"
        bl = BoxLayout(orientation = 'vertical', padding = 10)
        gl = GridLayout(cols = 4, spacing = 2, size_hint = (1, .6))

        self.lbl = Label(text='0', font_size = 40,
                         valign = "center", halign = "right",
                         size_hint = (1, .4),
                         text_size = (400 - 20, 500 * .4 - 20))
        bl.add_widget(self.lbl)

        gl.add_widget(Button(text='7',on_press = self.add_number))
        gl.add_widget(Button(text='8',on_press = self.add_number))
        gl.add_widget(Button(text='9',on_press = self.add_number))
        gl.add_widget(Button(text='x',on_press = self.add_operation))

        gl.add_widget(Button(text='4',on_press = self.add_number))
        gl.add_widget(Button(text='5',on_press = self.add_number))
        gl.add_widget(Button(text='6',on_press = self.add_number))
        gl.add_widget(Button(text='-',on_press = self.add_operation))

        gl.add_widget(Button(text='1',on_press = self.add_number))
        gl.add_widget(Button(text='2',on_press = self.add_number))
        gl.add_widget(Button(text='3',on_press = self.add_number))
        gl.add_widget(Button(text='+',on_press = self.add_operation))

        gl.add_widget(Button(text="C",on_press = self.delete_number))
        gl.add_widget(Button(text='0',on_press = self.add_number))
        gl.add_widget(Button(text='.',on_press = self.add_number))
        gl.add_widget(Button(text='=',on_press = self.calc_result))

        bl.add_widget(gl)
        return bl

if __name__ == '__main__':
    CalculatorApp().run()