[Python] Why does my approach to destroy window fail?

I want the EXIT button to close the window but I get the following error:

self.button=ttk.Button(self, text=' EXIT', command=app.exit).grid(column=0, row=0, sticky='ew') NameError: name 'app' is not defined. Did you mean: 'App'?

Can someone tell me why and how to fix it please?

#!/usr/bin/python3.9

import tkinter as tk
from tkinter import ttk

class ButtonFrame(ttk.Frame):
def __init__(self, container):
super().__init__(container)

self.__create_widgets()

def __create_widgets(self):

self.button=ttk.Button(self, text=' EXIT', command=app.exit).grid(column=0, row=0, sticky='ew')
self.grid(padx=0, pady=0)

class App(tk.Tk):
def __init__(self):
super().__init__()

self.configure(bg='#ff3800')
self.geometry("500x200") # wxh
self.resizable(False, False)

# layout on the root window
self.columnconfigure(0, weight=1)

self.__create_widgets()

def __create_widgets(self):

# create the button frame
button_frame = ButtonFrame(self)
button_frame.grid(column=0, row=0)

def exit(self):
self.destroy()


if __name__ == "__main__":

app = App()

app.mainloop()

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