Array index is out of range. error

this is my code : public class MainMenu_script1 : MonoBehaviour {

 //This public variable will hold a reference to your button image.
 public Texture2D signOutButton;
 
 public Texture2D[] buttonTextures;
 private float buttonWidth;
 private float buttonHeight;
 
 void OnGUI(){
     for (int i = 0; i < 2; i++) {
     ===>    if (GUI.Button (new Rect ((float)Screen.width * 0.5f - (buttonWidth / 2),
                                   (float)Screen.height * (0.6f + (i * 0.2f)) - (buttonHeight / 2),
                                   buttonWidth ,  buttonHeight), buttonTextures[i])) {
             Debug.Log("Mode " + i + " was clicked!");
             
             if (i == 0) {
                 // Single player mode!
                 RetainedUserPick_script.Instance.multiplayerGame = false;
                 //Application.LoadLevel("PickCarMenu");
             } else if (i == 1) {
                 RetainedUserPick_script.Instance.multiplayerGame = true;
                 MultiplayerController_script.Instance.SignInAndStartMPGame();
             }
         }
     }//end of for()
     /*This code checks if there is currently a signed-in user; if so, 
      * it displays a button to sign the user out. If the player taps that button,
      * the app calls SignOut() on your MultiplayerController_script.
      */
     if (MultiplayerController_script.Instance.IsAuthenticated()) {
         if (GUI.Button(new Rect(Screen.width  - (buttonWidth * 0.75f),
                                 Screen.height - (buttonHeight * 0.75f),
                                 buttonWidth * 0.75f,
                                 buttonHeight * 0.75f),signOutButton)) 
             {
             MultiplayerController_script.Instance.SignOut();
         }
     }//end of if MultiPlayerController_script
     
     MultiplayerController_script.Instance.SignInAndStartMPGame();
     
 }//end of OnGUI();
 
 
 // Use this for initialization
 void Start () {

     // I know that 301x55 looks good on a 660-pixel wide screen, so we can extrapolate from there
     buttonWidth = 301.0f * Screen.width / 660.0f;
     buttonHeight = 55.0f * Screen.width / 660.0f;

     Screen.sleepTimeout = SleepTimeout.NeverSleep;
     MultiplayerController_script.Instance.TrySilentSignIn();
     
 }//end of Start();

}//end of class

it show me the error on the first If statment on for loop could anyone help me hhow can I fix it ?