Rigidbody2D.IsKinematic giving object reference not set?

In the game the player picks up parts from a hook and drops them. I can pick them up from the hook but when they're supposed to be dropped nothing happens and kinematic isn't turned off for some reason.

 using UnityEngine;
 using System.Collections;

 public class rocketPartInteraction : MonoBehaviour {

 public GameObject rocketpart;
 private GameObject hand;
 public bool holding;
 private bool pickedUp;
 private Rigidbody2D rocketrigid;

 // Use this for initialization
 void Start () {
     holding = false;
     pickedUp = false;
     hand = GameObject.FindWithTag("Player");
     Rigidbody2D rocketrigid = this.GetComponent<Rigidbody2D>();
     rocketrigid.isKinematic = true;
 }
 
 // Update is called once per frame
 void FixedUpdate () {

     if (holding) {
         this.rocketpart.transform.position = hand.transform.position + new Vector3(1, 3, 0);
     }

     if (pickedUp)
         rocketrigid.isKinematic = false;   <--- this is the problem
 }

 void OnTriggerStay2D(Collider2D other){

     if (other.gameObject.tag == "Player") {

         if (Input.GetKeyDown ("space") && !holding) {
             //Debug.Log ("Pickup Works");
             holding = true;
         }

         else if (Input.GetKeyDown ("space") && holding) {
             holding = false;
             pickedUp = true;
         }
     }
 }
 }

NullReferenceException: Object reference not set to an instance of an object rocketPartInteraction.FixedUpdate () (at Assets/Scripts/rocketPartInteraction.cs:29)

also if anyone has a better suggestion on giving the item gravity that would be appreciated as well.