NullReferenceException: Object reference not set to an instance of an object PlayStream.OnMouseDown

Hi

I am building an AR app with vuforia and I want to do something like the code below but I get the " NullReferenceException: Object reference not set to an instance of an object PlayStream.OnMouseDown () (at Assets/NonAR-Assets/Scripts/PlayStream.cs:18) UnityEngine.SendMouseEvents:DoSendMouseEvents(Int32, Int32) " error. is there any suggestions ?

using UnityEngine; using System.Collections;

public class PlayStream : MonoBehaviour {

 public string url;

 // Use this for initialization
 void Start () 
 {
     WWW www = new WWW ("http://dl.pop-music.ir/music/1393/Mehr/Alireza%20Talischi%20-%20Nemikhamet%20%20%5B128%5D.mp3");
     audio.clip = www.audioClip;
     audio.clip = www.GetAudioClip (true, true);

 }
 
 // Update is called once per frame
 void OnMouseDown () {
     if (!audio.isPlaying && audio.clip.isReadyToPlay) {
         audio.Play ();
     } 
     else 
     {
         audio.Stop();
     }
 }

}