Attempt to call getDuration in wrong state: mPlayer=0x0, mCurrentState=1 How can I solve it?

Question

I am a novice developer trying to make an audio player on android.

On Youtube (https://www.youtube.com/watch?v=hNbXrlrWzGY&list=PL9vy4y29rrd4x5pAbowit8gpjsXAai0yF