Problem when trying to make a circular button on kivy

from kivy.uix.button import Button
from kivy.uix.behaviors.button import ButtonBehavior
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.image import Image
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.clock import Clock
from kivy.uix.widget import Widget
from kivy.graphics import BorderImage
from kivy.graphics import Color, RoundedRectangle, Rectangle, Triangle
from kivy.core.window import Window
#from kivy.config import Config
from kivy.uix.checkbox import CheckBox 
from kivy.uix.popup import Popup
from kivy.uix.scrollview import ScrollView
from kivy.uix.filechooser import FileChooserListView
from kivy.properties import ObjectProperty


class OpeningPage(FloatLayout):
    def __init__(self, **kwargs): 
        super().__init__(**kwargs)
        
        
        self.but = RoButton(text = 'START', pos = (350,100), font_size=14, size=(100,60), size_hint=(None,None))
        self.add_widget(self.but)
            
                   
class RoButton(Button):
    butt = ObjectProperty()
  
    def __init__(self, **kwargs): 
        super(RoButton, self).__init__(**kwargs)
        text = self.text
        with self.canvas:
           # Color(1., 0, 0)
            self.butt = RoundedRectangle( size= self.size, pos = self.pos, radius =[400])
                      

class UI(App):
    def build(self):
        
        self.screen_manager = ScreenManager()
        
        self.opening_page = OpeningPage()
        screen = Screen(name ='Opening_Page')
        screen.add_widget(self.opening_page)
        self.screen_manager.add_widget(screen)
         
        return self.screen_manager



if __name__ == '__main__':
    the_app = UI()
    the_app.run()   

Everytime I try to make a circular button using this code I get a box behind the circle. I tried doing self.canvas.before but still no luck if possible could answers be in python rather than .kv language thanks. Attached image of problem]1