widgetexceptionerror i am trying to build a simple app using python kivy and this is the error which i am getting - kivy

Question

I am trying to build a app using kivy and I am getting a widget expression error
raise WidgetException('Cannot add %r, it already has a parent %r'
kivy.uix.widget.WidgetException: Cannot add <kivy.uix.button.Button object at 0x0000026C4F999E40>, it already has a parent <main.SpartanGrid object at 0x0000026C4F1480B0>
this is my code
import turtle
import kivy
from kivy.app import App
from kivy.uix.gridlayout import GridLayout
from kivy.uix.label import Label
from kivy.uix.textinput import TextInput
from kivy.uix.button import Button
wn = turtle.Screen()
wn.setup(height=1.0,width=1.0)
wn.bgcolor('black')
turtle.hideturtle()
usr = turtle.textinput("what user wish to", "please tell weather you want to access or
write to the file")
usr.lower()
if usr == 'access':
with open("C:\\Users\\Badri\\Documents\\New folder\\pyproject.txt", 'r') as pr:
cont = pr.read()
turtle.color('red')
turtle.write(cont, align='center', font=('caliber', 25, 'normal'))
elif usr == 'write':
class SpartanGrid(GridLayout):
def __init__(self,**kwargs):
super(SpartanGrid,self).__init__()
self.cols=3
self.add_widget(Label(text="person Name:"))
self.s_name=TextInput()
self.add_widget(self.s_name)
self.add_widget(Label(text="person ph.no"))
self.s_no = TextInput()
self.add_widget(self.s_no)
self.press = Button(text="submit")
self.add_widget(self.press)
self.press.bind(on_press=self.submit)
self.add_widget(self.press)
def submit(self,instance):
print("successful")
class SpartanApp(App):
def build(self):
return SpartanGrid()
if __name__ == "__main__":
SpartanApp().run()
else:
turtle.write("the input is not valid",align='center',font=('caliber',25,'normal'))
turtle.bye()
turtle.done()