Python/Kivy App 只运行一次

I'm running a very simple python (3.5) script using Spyder IDE from Anaconda platform.

from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.gridlayout import GridLayout
from kivy.uix.textinput import TextInput

class LoginScreen(GridLayout):
    def __init__(self, **kwargs):
        super(LoginScreen,self).__init__(*kwargs)
        self.cols = 2
        self.add_widget(Label(text='Username'))
        self.username = TextInput(multiline=False)
        self.add_widget(self.username)
    
        self.add_widget(Label(text='Password'))
        self.password = TextInput(multiline=False, password=True)
        self.add_widget(self.password)

class TestApp(App):
    def build(self):
        return LoginScreen()

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

应用程序"第一次尝试运行良好(它还没有做任何事情，只是启动)，但是当我尝试再次启动它时，我收到以下错误消息: