小编sii*_*taw的帖子

当我运行此代码时，它显示“ TypeError: 'NoneType' object is not subscriptable ”。当我单击按钮时，我想切换到其他屏幕我正在制作一个小程序，但遇到了错误“TypeError：‘NoneType’对象不可下标。我以前从未见过此错误，所以我不知道是什么它的意思是。

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager , Screen


#define our different screens

class StartTestBtn(Screen):
     pass

class TestsResultBtn(Screen):
     pass

class SettingBtn(Screen):
     pass

class ContactUsBtn(Screen):
     pass

class MainPage(Screen):
     pass

class WindowManager(ScreenManager):
     pass


class MyGridLayout(Widget):
     pass


Builder.load_string("""
#:import utils kivy.utils
WindowManager:
     MainPage:
     StartTestBtn:
     TestsResultBtn:
     SettingBtn:
     ContactUsBtn:


<MainPage>:
     name:"main page"

     BoxLayout:
          cols:1
          size: root.width , root.height
          spacing: 20
          padding: 150

          Label:
               text: "Welcom"
               font_size:72
  
          Button:
               text: …