Need help with iTween manual skip and device hang

Hello all! I have been struggling for about a week with this problem. I make comic animations using iTween. I need some help on fixing this code. The idea is that the user can skip/speed up the comic by tapping. I stop the current iTween and set it manually at the final position, and just run the next iTween. The problem: the iPad or iPod device sometimes hangs when I try to skip the comics. Here is the code I'm using:

 #pragma strict
 #pragma downcast
 public var fadeInTime:float;
 public var timePerFrame:float = 2;
 public var plane1: GameObject;
 public var slideSpeed: float = 25;
 public var scaleSpeed: float = 1;
 public var canSkip: boolean = true;
 private var plane1Scale: Vector3;
 private var currentFrame: int=0;
 
 function Start() {
     iTween.CameraFadeAdd();    
     iTween.CameraFadeFrom({
         "amount":1,
         "time":fadeInTime,
         "onComplete":"comicsStart",
         "onCompleteTarget":transform.gameObject
     });
 }
 function Awake() {
     plane1Scale=plane1.transform.localScale;
 }
 function Update() {
     getInput();
 }
 
 function comicsStart() {
         //Move and scale down from right
         currentFrame=1;
         iTween.CameraFadeDestroy();
         plane1.renderer.enabled=true;
         iTween.MoveFrom(plane1,{
             "x":55,
             "time":fadeInTime,
             "delay":1,
             "easetype":"easeOutCirc",
             "speed":slideSpeed,
             "onComplete":"showPanel2",
             "onCompleteParams":timePerFrame,
             "onCompleteTarget":transform.gameObject
         });
         iTween.ScaleFrom(plane1,{
             "x":5.7,
             "y":3,
             "z":4.4,
             "delay":1,
             "easetype":"easeOutSine",
             "speed":scaleSpeed/2
         });
     }
 
 function showPanel2() {
     //Move away and scale down
     currentFrame=2;
     iTween.MoveTo(plane1,{
                 "x":-55,
                 "time":fadeInTime,
                 "delay":timePerFrame,
                 "easetype":"easeInCirc",
                 "speed":slideSpeed,
                 "onComplete":"fadeOut",
                 "onCompleteParams":0,
                 "onCompleteTarget":transform.gameObject
             });
         iTween.ScaleTo(plane1,{
                 "x":0,
                 "y":0,
                 "z":0,
                 "delay":timePerFrame,
                 "easetype":"easeInSine",
                 "speed":scaleSpeed*2
             });
 }
         
 function fadeOut(tempDelay:float) {
     currentFrame=3;
     iTween.CameraFadeAdd();
     iTween.CameraFadeTo({
         "amount":1,
         "time":fadeInTime,
         "delay":tempDelay*2,
         "onComplete":"endComics",
         "onCompleteTarget":transform.gameObject
     });
         
 }
     
 function endComics() {
     Application.LoadLevel(Scenes.WORLD_MAP);
 }
 
 function getInput() {
     if (canSkip==true) {
         if (CursorUtils.hitState() == HitState.Down) {
             skipComics();
         }
     }
 }            
 
 function skipComics() {
     switch (currentFrame) {
         case 1 :
             iTween.Stop(plane1);
             plane1.transform.position=Vector3(0,0,0);
             plane1.transform.localScale=plane1Scale;
             timePerFrame=0.5;
             showPanel2();
             break;
         case 2 :
             iTween.Stop(plane1);
             fadeOut(0);
             break;
     }    
 }

When I leave the animation to play without interrupting it with iTween.Stop() or iTween.StopByName() it works on the iPad. When I try to skip it sometimes just hangs and I recieve the following in Xcode:

 (Filename:  Line: -1)
 
     NullReferenceException
       at iTween.Stop (UnityEngine.GameObject target) [0x00000] in <filename unknown>:0 
       at iTween.Stop () [0x00000] in <filename unknown>:0 
       at ShaolinDepart_Comics01_Script.skipComics () [0x00000] in <filename unknown>:0 
       at ShaolinDepart_Comics01_Script.getInput () [0x00000] in <filename unknown>:0 
       at ShaolinDepart_Comics01_Script.Update () [0x00000] in <filename unknown>:0 

..which I don't know what it means.

I use Unity 3.3 and Xcode 4.2.

Thanks for your help in advance and I need this ASAP. Thanks!