为什么使用Kivy的时候报错： TypeError: 'NoneType' object is not subscriptable？

在写kivy的时候报了以下错误：

Traceback (most recent call last):
   File "C:\Users\Yeee\Desktop\import kivy.py", line 21, in <module>
     My().run()
   File "C:\Users\Yeee\AppData\Local\Programs\Python\Python310\lib\site-packages\kivy\app.py", line 954, in run
     self._run_prepare()
   File "C:\Users\Yeee\AppData\Local\Programs\Python\Python310\lib\site-packages\kivy\app.py", line 923, in _run_prepare
     self.load_kv(filename=self.kv_file)
   File "C:\Users\Yeee\AppData\Local\Programs\Python\Python310\lib\site-packages\kivy\app.py", line 696, in load_kv
     root = Builder.load_file(rfilename)
   File "C:\Users\Yeee\AppData\Local\Programs\Python\Python310\lib\site-packages\kivy\lang\builder.py", line 305, in load_file
     return self.load_string(data, **kwargs)
   File "C:\Users\Yeee\AppData\Local\Programs\Python\Python310\lib\site-packages\kivy\lang\builder.py", line 372, in load_string
     parser = Parser(content=string, filename=fn)
   File "C:\Users\Yeee\AppData\Local\Programs\Python\Python310\lib\site-packages\kivy\lang\parser.py", line 483, in __init__
     self.parse(content)
   File "C:\Users\Yeee\AppData\Local\Programs\Python\Python310\lib\site-packages\kivy\lang\parser.py", line 593, in parse
     objects, remaining_lines = self.parse_level(0, lines)
   File "C:\Users\Yeee\AppData\Local\Programs\Python\Python310\lib\site-packages\kivy\lang\parser.py", line 696, in parse_level
     _objects, _lines = self.parse_level(
   File "C:\Users\Yeee\AppData\Local\Programs\Python\Python310\lib\site-packages\kivy\lang\parser.py", line 756, in parse_level
     if current_property[:3] == 'on_':
 TypeError: 'NoneType' object is not subscriptable

然鹅我的代码是这样的：
这是.py文件的代码：

import kivy
from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.gridlayout import GridLayout
from kivy.uix.textinput import TextInput
from kivy.uix.button import Button
from kivy.uix.widget import Widget
#from kivy.properties import ObjectProperty

class MyGrid(Widget):
    pass

class My(App):
    def build(self):
        return MyGrid()

if __name__ == "__main__":
    My().run()

我的.kv代码是这样的：

<MyGrid>:
    GridLayout:
        cols:1

        size: root.width, root.height

            GridLayout:
            cols:2

            Label:
                text: "input file position"

            TextInput:
                multiline:False

            Label:
                text: "output file position"

            TextInput:
                multiline:False


        Button:
            text: "Start"

完全不知道发生了什么啊有没有（一脸懵逼

这个错误是指你current_property变量的值是None