[Python] python Tkinter- Close thread on window close(x)

How do I kill an already running thread when tkinter root window is closed. I am able to detect events but if the user has not started any thread I run into an error while closing the window. Also when closing if I am able to detect thread is running with function

self.thread.is_alive()

what command is used to kill the thread?

import threading
import tkinter as tk
from tkinter import messagebox
import time


class Application(tk.Frame):
def __init__(self, master=None):
tk.Frame.__init__(self, master)
self.pack()
self.createWidgets()
self.master.title("myapp")
master.protocol("WM_DELETE_WINDOW", self.on_closing)

def createWidgets(self):
self.btn = tk.Button(self)
self.btn["text"] = "Start.."
self.btn.pack()
self.btn["command"] = self.startProcess

def startProcess(self):
self.thread = threading.Thread(target=self.helloWorld, args=("Hello World",))
self.thread.start()

def helloWorld(self, txt):
for x in range(5):
print (txt)
time.sleep(5)


def on_closing(self):
if messagebox.askokcancel("myapp", "Do you want to quit?"):
if self.thread.is_alive():
self.thread.stop()
self.master.destroy()

def main():
root = tk.Tk()
app = Application(master=root)
app.mainloop()


if __name__ == "__main__":
main()

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