Не выводится кнопка

[Karina. 234567]

У меня программа что то наподобие рисовалки, помогите пожалуйста почему не выводится на экран кнопка black

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.button import Button

from random import random
from kivy.core.window import Window
from kivy.graphics import (Color, Ellipse, Rectangle, Line)
from kivy.uix.gridlayout import GridLayout


class PainterWidget(Widget):
    color = (0.2, 0, 1, 1)

    def on_touch_down(self, touch):
        with self.canvas:
            Color(rgba=self.color)
            touch.ud["line"] = Line(points=(touch.x, touch.y), width=15)

    def on_touch_move(self, touch):
        touch.ud["line"].points += (touch.x, touch.y)
  
        
class PaintApp(App):

    def build(self):
        bl = GridLayout(cols=1, padding=[350])

        bl.add_widget( self.lbl )

        gl.add_widget( Button(text="black", on_press = self.black, size = (100, 50), pos = (600, 0)))
        
        return bl
 

    def build(self):
 
        parent = Widget()
        self.painter = PainterWidget()
        parent.add_widget(self.painter)


        parent.add_widget(Button(text = 'Clear' , on_press = self.clear_canvas, size = (100, 50)))
        parent.add_widget(Button(text = 'Save' , on_press = self.save, size = (100, 50), pos = (100, 0)))
        parent.add_widget(Button(text = 'Screen', on_press = self.screen, size = (100, 50), pos=(200, 0 )))
        parent.add_widget(Button(text = 'red', on_press = self.red, size = (100, 50), pos=(300, 0 )))
        parent.add_widget(Button(text = 'white', on_press = self.white, size = (100, 50), pos=(400, 0 )))
        parent.add_widget(Button(text = 'blue', on_press = self.blue, size = (100, 50), pos=(500, 0 )))

        return parent
    
    def red(self, instance):
        self.painter.color = (1, 0, 0.2, 1)

    def blue(self, instance):
        self.painter.color = (0.2, 0, 1, 1)   

    def white(self, instance):
        self.painter.color = (1, 1, 1, 1)
    
    def clear_canvas(self, instance):
        self.painter.canvas.clear()   

    def save(self, instance):
        self.painter.size = (Window.size[0], Window.size[1])
        self.painter.export_to_png('mage.png')

    def screen(self, instance):
        Window.screenshot('screen.png')       
        
if __name__ == '__main__':
    PaintApp().run()