AttributeError: 'NoneType' object has no attribute 'is_using'

[ShifeiDuan]

ISSUE TEMPLATE ver. 1.2.0

Please fill this template for more accurate and prompt support.

1. Which DYNAMIXEL SDK version do you use?
   3.7.42

2. Which programming language/tool do you use?
   Python`

3. Which operating system do you use?
   Ubuntu 20.04`

4. Which USB serial converter do you use?
   U2D2

5. Which DYNAMIXEL do you use?
   XM430-W250

6. Have you searched the issue from the closed issue threads?
   Yes

7. Please describe the issue in detail
   When I use ROS to send the motor position to the motor, I got error: File "/opt/ros/noetic/lib/python3/dist-packages/dynamixel_sdk/protocol2_packet_handler.py", line 213, in txPacket
   if port.is_using:
   AttributeError: 'NoneType' object has no attribute 'is_using'

8. How can we reproduce the issue?