The code from the official site does not work! Pythonkivy! What to do?

Question

Python

<PongBall>:
    size: 50, 50 
    canvas:
        Ellipse:
            pos: self.pos
            size: self.size          

<PongPaddle>:
    size: 25, 200
    canvas:
        Rectangle:
            pos:self.pos
            size:self.size

<PongGame>:
    ball: pong_ball
    player1: player_left
    player2: player_right
    
    canvas:
        Rectangle:
            pos: self.center_x-5, 0
            size: 10, self.height
    
    Label:
        font_size: 70  
        center_x: root.width / 4
        top: root.top - 50
        text: str(root.player1.score)
        
    Label:
        font_size: 70  
        center_x: root.width * 3 / 4
        top: root.top - 50
        text: str(root.player2.score)
    
    PongBall:
        id: pong_ball
        center: self.parent.center
        
    PongPaddle:
        id: player_left
        x: root.x
        center_y: root.center_y
        
    PongPaddle:
        id: player_right
        x: root.width-self.width
        center_y: root.center_y

<PongBall>:
    size: 50, 50 
    canvas:
        Ellipse:
            pos: self.pos
            size: self.size          

<PongPaddle>:
    size: 25, 200
    canvas:
        Rectangle:
            pos:self.pos
            size:self.size

<PongGame>:
    ball: pong_ball
    player1: player_left
    player2: player_right
    
    canvas:
        Rectangle:
            pos: self.center_x-5, 0
            size: 10, self.height
    
    Label:
        font_size: 70  
        center_x: root.width / 4
        top: root.top - 50
        text: str(root.player1.score)
        
    Label:
        font_size: 70  
        center_x: root.width * 3 / 4
        top: root.top - 50
        text: str(root.player2.score)
    
    PongBall:
        id: pong_ball
        center: self.parent.center
        
    PongPaddle:
        id: player_left
        x: root.x
        center_y: root.center_y
        
    PongPaddle:
        id: player_right
        x: root.width-self.width
        center_y: root.center_y

Ошибка:
[INFO ] [Logger ] Record log in C:\Users\stepan\.kivy\logs\kivy_18-05-13_114.txt
[INFO ] [Kivy ] v1.10.0
[INFO ] [Python ] v3.6.5 (v3.6.5:f59c0932b4, Mar 28 2018, 16:07:46) [MSC v.1900 32 bit (Intel)]
[INFO ] [Factory ] 194 symbols loaded
[INFO ] [Image ] Providers: img_tex, img_dds, img_sdl2, img_gif (img_pil, img_ffpyplayer ignored)
[INFO ] [OSC ] using for socket
[INFO ] [Window ] Provider: sdl2
[INFO ] [GL ] Using the "OpenGL" graphics system
[INFO ] [GL ] GLEW initialization succeeded
[INFO ] [GL ] Backend used
[INFO ] [GL ] OpenGL version
[INFO ] [GL ] OpenGL vendor
[INFO ] [GL ] OpenGL renderer
[INFO ] [GL ] OpenGL parsed version: 4, 4
[INFO ] [GL ] Shading version
[INFO ] [GL ] Texture max size <16384>
[INFO ] [GL ] Texture max units <32>
[INFO ] [Shader ] fragment shader:
[INFO ] [Shader ] vertex shader:
[INFO ] [Window ] auto add sdl2 input provider
[INFO ] [Window ] virtual keyboard not allowed, single mode, not docked
[INFO ] [Base ] Start application main loop
[INFO ] [Base ] Leaving application in progress...
Traceback (most recent call last):
File "SnakeGame.py", line 73, in
PongApp().run()
File "C:\Users\stepan\AppData\Local\Programs\Python\Python36-32\lib\site-packages\kivy\app.py", line 828, in run
runTouchApp()
File "C:\Users\stepan\AppData\Local\Programs\Python\Python36-32\lib\site-packages\kivy\base.py", line 504, in runTouchApp
EventLoop.window.mainloop()
File "C:\Users\stepan\AppData\Local\Programs\Python\Python36-32\lib\site-packages\kivy\core\window\window_sdl2.py", line 663, in mainloop
self._mainloop()
File "C:\Users\stepan\AppData\Local\Programs\Python\Python36-32\lib\site-packages\kivy\core\window\window_sdl2.py", line 405, in _mainloop
EventLoop.idle()
File "C:\Users\stepan\AppData\Local\Programs\Python\Python36-32\lib\site-packages\kivy\base.py", line 339, in idle
Clock.tick()
File "C:\Users\stepan\AppData\Local\Programs\Python\Python36-32\lib\site-packages\kivy\clock.py", line 581, in tick
self._process_events()
File "kivy\_clock.pyx", line 367, in kivy._clock.CyClockBase._process_events (kivy\_clock.c:7700)
File "kivy\_clock.pyx", line 397, in kivy._clock.CyClockBase._process_events (kivy\_clock.c:7577)
File "kivy\_clock.pyx", line 395, in kivy._clock.CyClockBase._process_events (kivy\_clock.c:7498)
File "kivy\_clock.pyx", line 167, in kivy._clock.ClockEvent.tick (kivy\_clock.c:3490)
File "SnakeGame.py", line 39, in update
self.ball.move()
AttributeError: 'NoneType' object has no attribute 'move'
C:\Users\stepan\Desktop\Python Kivy>Cpython SnakeGame.py
[INFO ] [Logger ] Record log in C:\Users\stepan\.kivy\logs\kivy_18-05-13_115.txt
[INFO ] [Kivy ] v1.10.0
[INFO ] [Python ] v3.6.5 (v3.6.5:f59c0932b4, Mar 28 2018, 16:07:46) [MSC v.1900 32 bit (Intel)]
[INFO ] [Factory ] 194 symbols loaded
[INFO ] [Image ] Providers: img_tex, img_dds, img_sdl2, img_gif (img_pil, img_ffpyplayer ignored)
[INFO ] [OSC ] using for socket
[INFO ] [Window ] Provider: sdl2
[INFO ] [GL ] Using the "OpenGL" graphics system
[INFO ] [GL ] GLEW initialization succeeded
[INFO ] [GL ] Backend used
[INFO ] [GL ] OpenGL version
[INFO ] [GL ] OpenGL vendor
[INFO ] [GL ] OpenGL renderer
[INFO ] [GL ] OpenGL parsed version: 4, 4
[INFO ] [GL ] Shading version
[INFO ] [GL ] Texture max size <16384>
[INFO ] [GL ] Texture max units <32>
[INFO ] [Shader ] fragment shader:
[INFO ] [Shader ] vertex shader:
[INFO ] [Window ] auto add sdl2 input provider
[INFO ] [Window ] virtual keyboard not allowed, single mode, not docked
Traceback (most recent call last):
File "SnakeGame.py", line 74, in
PongApp().run()
File "C:\Users\stepan\AppData\Local\Programs\Python\Python36-32\lib\site-packages\kivy\app.py", line 802, in run
root = self.build()
File "SnakeGame.py", line 68, in build
game.serve_ball()
File "SnakeGame.py", line 36, in serve_ball
self.ball.center = self.center
AttributeError: 'NoneType' object has no attribute 'center'

Answer 1

All decided!
Here is the link to the answer - https://stackoverflow.com/questions/21713977/kivy-...
The kv file name didn't match the class