Changing texture for pause button

I am trying to changing the texture for a button when click pause/resume game.

The error is "UnassignedReferenceException: The variable button of PauseButton has not been assigned. You probably need to assign the button variable of the PauseButton script in the inspector. PauseButton.OnMouseDown () (at Assets/Scripts/PauseButton.cs:28) UnityEngine.SendMouseEvents:DoSendMouseEvents(Int32, Int32)"

The same apply to line 32 (when click the pause button again).

The code is following

 using UnityEngine;
 using System.Collections;
 
 public class PauseButton : MonoBehaviour {
 
     public static bool pauseGame = false;
     public static bool pauseButtonClick = false;
     public static Animator animator;
     
     private Texture2D play;
     private Texture2D pause;
     public GUITexture button;
 
     void Start () 
     {
         animator = GetComponent<Animator>();
         Texture play = Resources.Load("playbutton") as Texture2D;
         Texture pause = Resources.Load("pausebutton") as Texture2D;
     }
 
     void OnMouseDown()
     {
         animator.SetBool( "pauseButtonClick", true);
         Invoke ("EndPauseButtonAnimation", .2f);
         pauseGame =!pauseGame;
         if(pauseGame)
         {
             button.guiTexture.texture = play;
         }
         else
         {
             button.guiTexture.texture = pause;
         }
     }
 
     void EndPauseButtonAnimation()
     {
         animator.SetBool( "pauseButtonClick", false);
     }