fix exception in MODBUS timeout callback

app/src/modbus.py

@@ -108,6 +108,7 @@ class Modbus():
     def close(self):
         """free the queue and erase the callback handlers"""
         logging.debug('Modbus close:')
+        self.__stop_timer()
         self.rsp_handler = None
         self.snd_handler = None
         while not self.que.empty:
@@ -252,6 +253,7 @@ class Modbus():
         # logging.debug(f'Modbus stop timer {self}')
         if self.tim:
             self.tim.cancel()
+            self.tim = None
 
     def __timeout_cb(self) -> None:
         '''Rsponse timeout handler retransmit pdu or send next pdu'''