Thread not joining because of label.config

When I do label.config(text='changed'), it does not happen and stops the thread from closing. Due to it, the exit_window function is not working. The root window is becoming unresponsive and nothing is happening. I am using the latest python version. I have tried many things but it is not working. Please tell me where I am going wrong.

Code:

import tkinter
import threading
import time
import datetime

def format_time(secs: int) -> str:
    """Function that takes the total seconds and convert it to
    hours, minutes and seconds and then print them."""
    hours = secs // 3600
    mins = secs // 60
    secs = secs % 60
    return f"{hours:02}:{mins:02}:{secs:02}"

def save_time() -> None:
    global SECS
    # If the time is not zero meaning if the watch is not stopped.
    if SECS:
        current_d_t = datetime.datetime.now()
        filename = "Stopwatch " + current_d_t.strftime("%d-%M-%Y")
        with open(filename, "a") as f:
            f.write("TIME: " + format_time(SECS) + "\n\n")

def inc_stopwatch(time_label: tkinter.Label) -> None:
    # Get total seconds
    global SECS, IS_RUNNING
    IS_RUNNING = True
    while True:
        print(IS_RUNNING)
        SECS += 1
        time.sleep(1)
        time_label.config(text=format_time(SECS))
        if not IS_RUNNING:
            break
    print("Ended while loop.")

def start_time(time_label: tkinter.Label):
    global THREADS, IS_RUNNING
    if not THREADS:
        t = threading.Thread(target=inc_stopwatch, args=(time_label, ))
        t.start()
        THREADS.append(t)
    else:
        IS_RUNNING = True

def exit_window(root: object) -> None:
    global IS_RUNNING, THREADS
    IS_RUNNING = False
    time.sleep(2)
    print(IS_RUNNING)
    if THREADS:
        print(THREADS)
        THREADS[0].join()
    root.quit()

FONT = ('Areal', 20)

# Make the root window.
root = tkinter.Tk()

# Display the title.
root.title("Stopwatch")

# Set the screen size to 400 rows by 400 cols.
root.geometry('400x400')

# Total Seconds.
SECS = 0

# Boolean to start or stop the time.
IS_RUNNING = False

# Thread for increasing the time.
THREADS = []

# Label to display time.
time_label = tkinter.Label(root, text='00:00:00', font=FONT)
time_label.pack()

# Button to start stopwatch.
start_button = tkinter.Button(root, text='Start', font=FONT,
                              command=lambda: start_time(time_label))
start_button.pack()

# Button to pause stopwatch.
pause_button = tkinter.Button(root, text='Pause', font=FONT)
pause_button.pack()

# Button to reset time.
reset_button = tkinter.Button(root, text='Reset', font=FONT)
reset_button.pack()

# Button to exit window.
exit_button = tkinter.Button(root, text='Exit', font=FONT,
                            command=lambda: exit_window(root))
exit_button.pack()

root.mainloop()