Python GUI 独立线程计时

我在写一个GUI，里面需要添加实时的时间，于是我开了个线程用来计时。代码如下：

import time
from threading import Thread
import tkinter as tk
import tkinter.ttk as ttk
from tkinter import StringVar


class TestUI(tk.Tk):
    def __init__(self):
        super().__init__()
        self.w = 390
        self.h = 200
        self.x = (self.winfo_screenwidth() - self.w) / 2
        self.y = (self.winfo_screenheight() - self.h) / 2

        self.title("test")
        self.geometry('%dx%d+%d+%d' % (self.w, self.h, self.x, self.y))
        self.resizable(False, False)
        self.str_time = StringVar()  # for timing

        self.build_ui()

    def timing(self):
        while True:
            str_time = time.strftime('%Y-%m-%d  %H:%M:%S', time.localtime())
            self.str_time.set(str_time)
            time.sleep(1)

    def build_ui(self):
        frame_001 = ttk.Frame(self)
        timing_thread = Thread(target=self.timing)
        timing_thread.start()
        ttk.Label(frame_001, width=40, textvariable=self.str_time).pack()

        frame_001.pack()


if __name__ == '__main__':
    root = TestUI()
    root.mainloop()

运行后可以正常计时，但是每次关闭GUI，都会有下面的异常：

Exception in thread Thread-1 (timing):
Traceback (most recent call last):
  File "C:\Users\*****\AppData\Local\conda\conda\envs\PyDataAnalysis\lib\threading.py", line 1016, in _bootstrap_inner
    self.run()
  File "C:\Users\*****\AppData\Local\conda\conda\envs\PyDataAnalysis\lib\threading.py", line 953, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Users\*****\PycharmProjects\tp_integrity\ui_test.py", line 26, in timing
    self.str_time.set(str_time)
  File "C:\Users\*****\AppData\Local\conda\conda\envs\PyDataAnalysis\lib\tkinter\__init__.py", line 402, in set
    return self._tk.globalsetvar(self._name, value)
RuntimeError: main thread is not in main loop

请问这个是什么原因，应该怎么解决？

因为子线程没有在主线程退出时结束
可将子线程设置为守护线程，使其随主线程退出而结束，在timing_thread.start()前加上

timing_thread.setDaemon(True)

但这种在子线程中操作主线程ui组件的行为后续运行中可能会遇到意料之外的错误