Command motor driver via serial port

Hello, community.

I have some problems with my code. I’m very new to ardupilot code and have some knowledge of C/C++. I just want to send bytes to command my motor driver via serial protocol (port telem). The motor driver will take control the wheels with velocity control mode. I’ve modified the code in my forked repository here
The code is to test that it can send the bytes via TLEM port and I just evaluated by using serial terminal on my laptop (ex. Arduino’s serial terminal). I’ve used AP_ProximityBackendSerial.cpp as a reference. As you see in the code, I use gcs().send_text() to display string in MP. The string always shows “uart = null”. I think the problem is my “serial_manager.find_serial” function in the code. I don’t know how to use the function correctly please give me advice and how to fix it.