This ensures flush() is only called if the stream is open, and logs (but ignores) any exceptions during flush. This should prevent the "Bad file descriptor" error.

I see this error a lot on a rak unit, I dont know this is the way but .. you be the judge.

The double flush() is not the root cause; the real issue is that code is trying to use the serial port after it has been closed.
The error occurs both in close() (during flush()) and later in _writeBytes() (during write()), indicating the port is closed or invalid at those times.