Compiling error: [0x00000] in :0

I made my first game in unity using C#, game works perfect in Unity, but when I make "Build & Run" the game dosn't work. I check the "output_log.txt" and says this:

NullReferenceException: Object reference not set to an instance of an object at scr_menu.OnGUI () [0x00000] in :0

(Filename: Line: -1)

I don't know what are I'm doing wrong, please help.

This is the script: using UnityEngine; using System.Collections;

 public class scr_menu : MonoBehaviour {
 
 
     public AudioClip select;
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
 
 
     }
 
     void OnGUI () {
 
         if (GUI.Button (new Rect (Screen.width/2 - 75,Screen.height/2 + 35,150,50),"Play Game")) {
 
             audio.PlayOneShot (select);
             Application.LoadLevel ("level_selector");
         }
 
         if (GUI.Button (new Rect (Screen.width/2 - 75,Screen.height/2 + 90,150,50),"Options")) {
 
             audio.PlayOneShot (select);
             Application.LoadLevel ("options");
         }            
 
         if (GUI.Button (new Rect (Screen.width/2 - 75,Screen.height/2 + 145,150,50),"Exit Game")) {
 
             audio.PlayOneShot (select);
             StartCoroutine (exiting());
 
         }
     }
     IEnumerator exiting () {
 
         yield return new WaitForSeconds(2);
         Application.Quit();
     }
 }

I made the project in Unity 4.3.4f1