Turning a gameobject's gravity on/off

So here's my code:

 #pragma strict
 
 var coconutPrefab : GameObject;
 var cocoHolder : GameObject;
 var player : Transform;
 private var drawGUI = false;
 private var cocoClone : GameObject;
 
 
 function Start () {
         var cocoClone:GameObject = Instantiate(coconutPrefab, cocoHolder.transform.position, cocoHolder.transform.rotation);
         cocoClone.rigidbody.useGravity = false;
         cocoClone.rigidbody.isKinematic = true;
         cocoClone.transform.parent = cocoHolder.transform;
         player = null;
 }
 
 function Update () {
     if(drawGUI == true)
     {
         if(Input.GetKeyDown(KeyCode.E))
         {        
             var tree = cocoHolder.transform.parent;
             tree.animation.Play("TreeShake");
             cocoClone.rigidbody.useGravity = true;
               cocoClone.rigidbody.isKinematic = false; 
                drawGUI = false;
         }
     }
 }
 
 function OnTriggerEnter (theCollider : Collider)
 {
     if (theCollider.tag == "Player")
     {    
         player = GameObject.FindGameObjectWithTag("Player").transform;
         Debug.Log("Player has been found");
         drawGUI = true;
     }
 }
 
 function OnTriggerExit (theCollider : Collider)
 {
     if (theCollider.tag == "Player")
     {
         drawGUI = false;
         transform.collider.isTrigger = false;
     }
 }
 
 function OnGUI ()
 {
     if (drawGUI == true)
     {
         GUI.Box (Rect (Screen.width*0.5-100, 200, 200, 22), "Press E To Shake Tree");
     }
 
 }

When I press E in the Update, these two lines throw me a Null reference error:

         cocoClone.rigidbody.useGravity = true;
         cocoClone.rigidbody.isKinematic = false;

I also tried changing the gameobject name from those two lines to coconutPrefab, because I had read that when you instantiate a prefab, the variable for the prefab refers to the clone. It doesn't give me an error, but it also doesn't add gravity to the coconut when I press E either. So I'm stumped, what am I doing wrong?