[Python] Clicking Close button results in error popup from FreeSimpleGUI

I am using FreeSimpleGUI to create a to-go list interface. When I click on the "Exit" button it closes as normal, but when I click on the "Close" button it result in this popup, how do I resolve this please? This is my first time posting here, so I apologise for any formatting issues.

GUI Error Message

import functions
import FreeSimpleGUI as sg
import time
import os

if not os.path.exists("todos.txt"):
with open("todos.txt", "w") as file:
pass

sg.theme("Black")

clock = sg.Text('', key='clock')
label = sg.Text("Type in a to-do")
input_box = sg.InputText(tooltip="Enter to-do", key="todo")
add_button = sg.Button("Add")
list_box = sg.Listbox(values=functions.get_todos(), key='todos',
enable_events=True, size=[45, 10])
edit_button = sg.Button("Edit")
complete_button = sg.Button("Complete")
exit_button = sg.Button("Exit")

window = sg.Window('My To-Do App',
layout=[[clock],
[label],
[input_box, add_button],
[list_box, edit_button, complete_button],
[exit_button]],
font=('Helvetica', 20))

while True:
event, values = window.read(timeout=500)
window["clock"].update(value=time.strftime("%b %d, %Y %H:%M:%S"))
match event:
case "Add":
todos = functions.get_todos()
new_todo = values['todo'] + "\n"
todos.append(new_todo)
functions.write_todos(todos)
window['todos'].update(values=todos)

case "Edit":
try:
todo_to_edit = values['todos'][0]
new_todo = values['todo'] + "\n"

todos = functions.get_todos()
index = todos.index(todo_to_edit)
todos[index] = new_todo
functions.write_todos(todos)
window['todos'].update(values=todos)
except IndexError:
sg.popup("Please select an item first", font=("Helvetica", 20))

case "Complete":
try:
todo_to_complete = values['todos'][0]
todos = functions.get_todos()
todos.remove(todo_to_complete)
functions.write_todos(todos)
window['todos'].update(values=todos)
window['todo'].update(value='')
except IndexError:
sg.popup("Please select an item first", font=("Helvetica", 20))

case "Exit":
break
case 'todos':
window['todo'].update(value=values['todos'][0])
case sg.WIN_CLOSED:
break

window.close()

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