具有ScreenManager的Kivy应用程序崩溃，因为WindowSDL对象没有属性“Manager”？

Question

我正在用Kivy和Python构建一个应用程序，并且正在我的屏幕之间添加导航。目前，该应用程序在点击欢迎页面时崩溃

以下是我的屏幕管理器的代码结构，以及屏幕：

class SwitchingScreenApp(App):

    def build(self):
        sm = ScreenManager()
        sm.add_widget(WelcomeScreen(name="welcome"))
        sm.add_widget(SignupScreen(name="signup"))
        sm.add_widget(ThanksScreen(name="thanks"))
        sm.current = "welcome"
        return sm

class WelcomeScreen(Screen):

    def __init__(self, **kwargs):
        super(WelcomeScreen, self).__init__(**kwargs)
        box = BoxLayout(orientation='vertical')
        welcomeLabel = Label(text='Welcome!',
            halign= 'center', font_size=32, color=(0,0,0,1))
        image = Image(source="logo.png", allow_stretch=True, keep_ratio=True)
        nextLabel = Label(text='Touch anywhere to proceed.',
            halign= 'center', font_size=24, color=(0,0,0,1))
        box.add_widget(welcomeLabel)
        box.add_widget(image)
        box.add_widget(nextLabel)
        self.add_widget(box)

    def callback(self):
        print('The button has been pressed')
        self.manager.current = 'signup'

SwitchingScreenApp().run()

目前，当我触摸屏幕时，应用程序崩溃，原因是

File "/home/pi/Terminal2/main.py", line 350, in on_motion
     self.manager.current = 'success'
 AttributeError: 'WindowSDL' object has no attribute 'manager'

我不知道还有什么地方可以看，因为我遵循了这篇文章的答案：python- Kivy Screen Manager within .py file

我的实现中缺少了什么