How can I assign a null default value for MediaRecorder.OnInfoListener?

Question

The Code A can work well, I hope to assign a null default value for MediaRecorder.OnInfoListener.

But both Code B and Code C don't work, how can I fix it ?

Code A

    fun startRecord(filename:String, mOnInfoListener: MediaRecorder.OnInfoListener){
        mRecorder = MediaRecorder()
        mRecorder?.let {
            with(it) {              
                setOnInfoListener(mOnInfoListener)
   
        }
    }


    public interface OnInfoListener
    {       
        void onInfo(MediaRecorder mr, int what, int extra);
    }

Code B

fun startRecord(filename:String, mOnInfoListener: MediaRecorder.OnInfoListener={ }){
       ...
}

Code C

fun startRecord(filename:String, mOnInfoListener: MediaRecorder.OnInfoListener=null){
       ...
}