fix: no old signal handler

app.py

@@ -13,7 +13,8 @@ def sigterm_handler_wrap(_signo):
     def func(_signo, _stack_frame):
         logger.info("signal {} received, exiting...".format(_signo))
         conf().save_user_datas()
-        return old_handler(_signo, _stack_frame)
+        if callable(old_handler): #  check old_handler
+            return old_handler(_signo, _stack_frame)
     signal.signal(_signo, func)
 
 def run():