The program does not work, everything seems to be written correctly, but it gives an error?

Question

EAV

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.properties import (
NumericProperty, ReferenceListProperty, ObjectProperty
)
from kivy.vector import Vector
from kivy.clock import Clock
from random import randint

class PongBall(Widget):

# Speed movement of our ball along two axes
velocity_x = NumericProperty(0)
velocity_y = NumericProperty(0)

# Create a conditional vector
velocity = ReferenceListProperty(velocity_x, velocity_y)

# Make the ball move
def move(self):
self.pos = Vector(*self.velocity ) + self.pos

class PongGame(Widget):
ball = ObjectProperty(None) # this will be our relation to the ball object

def serve_ball(self):
self.ball.center = self.center
self.ball.velocity = Vector(4, 0).rotate(randint(0, 360) )

def update(self, dt):
self.ball.move() # move the ball on every screen update

# bounce the ball along the y axis
if(self.ball.y < 0) or (self.ball.top > self.height ):
self.ball.velocity_y *= -1 # invert current y speed

# bounce ball on x
if(self.ball.x < 0) or (self.ball.right > self.widht):
self.ball .velocity_x *= -1 # invert current x speed

class PongApp(App):
def build(self):
game = PongGame()
game.serve_ball()
Clock.schedule_interval(game.update, 1.0/60) # FPS
return game

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

Error: AttributeError: 'NoneType' object has no attribute ' move'