PyQT5 多线程 程序关闭时报错RuntimeError: wrapped C/C++ object of type QTimer has been deleted

学习PyQT5时，例子程序:Chapter07:qt07_signalSlotThreaad
程序运行没问题，但是运行以后，关闭或者stop，均报错：

RuntimeError: wrapped C/C++ object of type QTimer has been deleted

程序代码：

#-*- encoding: gb2312 -*-
import string, threading, time
from PyQt5.QtCore import QThread ,  pyqtSignal,  QDateTime 
from PyQt5.QtWidgets import QApplication,  QDialog,  QLineEdit
import time
import sys
def thread_main(a):
    global count, mutex
    # 获得线程名
    threadname = threading.currentThread().getName()
 
    for x in range(0, int(a)):
        # 取得锁
        mutex.acquire()
        count = count + 1
        # 释放锁
        mutex.release()
        print (threadname, x, count)
        time.sleep(1)

def showthetime():
    while True:
        data = QDateTime.currentDateTime()
        currTime = data.toString("yyyy-MM-dd hh:mm:ss")
        #self.update_date.emit( str(currTime) )
        time.sleep(1)
        print(currTime)
 
def main(num):
    
    global count, mutex
    threads = []
 
    count = 1
    # 创建一个锁
    mutex = threading.Lock()
    # 先创建线程对象
    for x in range(0, num):
        threads.append(threading.Thread(target=thread_main, args=(10,)))
    # 启动所有线程
    for t in threads:
        t.start()
 
if __name__ == '__main__':

    num = 4
    # 创建4个线程
    main(4)
    t1 = threading.Thread(target=showthetime, args=())
    t1.setDaemon(True)
    t1.start()
    sys.exit()