[Python] Turtle Subclass "object has no attribute '_shown'" [duplicate]

New to Python here. I am messing around with Turtle module, and when I try to create a Turtle type class the following way I get the error message:

AttributeError: 'Buttons' object has no attribute '_shown'


My Python script:

import turtle

wn = turtle.Screen()
wn.bgcolor("black")
wn.setup(width=800, height=600)
wn.tracer(0)

class Buttons(turtle.Turtle):
def __init__(self, color, text):
self.hideturtle()
self.color(color)
self.shape('square')
self.shapesize(stretch_wid=3, stretch_len=7)
self.penup()
self.goto(0, 0)
self.stamp()
self.color('white')
self.goto(0, 0 - FONT_SIZE/2)
self.write(text, align='center', font=FONT)


t1 = Buttons("green", "start")

while True:
wn.update()

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