Object Reference not set to an instance of an object for Moving Platform

When I try to enter a trigger on a moving platform to let the platform move, it says "Object reference not set to an instance of an object" when I enter the trigger. Here is my code:

using UnityEngine; using System.Collections;

public class Finals : MonoBehaviour {

 void OnTriggerEnter2D(Collider2D other) {
     if (other.tag == "Player") {
         StartMove ();
     }
 }

 public void StartMove() {
     iTween.MoveTo(gameObject, iTween.Hash("x", transform.position.x + 20, "time", 20, iTween.EaseType.easeInOutSine,"delay",1));
 }

}