Linking projectiles to enemy

Hi, I want to be able to link the script of my projectiles to the enemy hands, so that they can damage them, but after you destroy one hand, you get a null reference exception, so i'm wondering how you do it without getting that, or even a work around, thanks.

projectile script:

     public class ProjectileController : MonoBehaviour {

 private Rigidbody2D rb2d;
 private Boss boss;
 private Hands hand1;
 private Hands hand2;

 public int damage;

 public float speed;


 // Use this for initialization
 void Start () {
 
     rb2d = GetComponent<Rigidbody2D> ();
     boss = GameObject.FindGameObjectWithTag ("Boss").GetComponent<Boss>();
     hand1 = GameObject.FindGameObjectWithTag ("Hand1").GetComponent<Hands>();
     hand2 = GameObject.FindGameObjectWithTag ("Hand2").GetComponent<Hands> ();
 }
 
 // Update is called once per frame
 void Update () {

     rb2d.velocity = new Vector2 (speed, rb2d.velocity.y);
 }

 void OnTriggerEnter2D(Collider2D col){
     if (col.CompareTag ("Hand1")) {
         hand1.Damage (damage);
         boss.Damage (damage);
         Destroy (this.gameObject);
     }

     if (col.CompareTag ("Hand2")) {
         hand2.Damage (damage);
         boss.Damage (damage);
         Destroy (this.gameObject);
     }

     if (col.CompareTag ("Boss")) {
         boss.Damage (damage);
         Destroy (this.gameObject);
     }
 }

}

boss script:

     public class Boss : MonoBehaviour {

 private Player player;
 private Rigidbody2D rb2d;
 private Animator anim;

 public float maxHealth;
 public float currHealth;
 public float handsHealth;

 public float rageLimit;
 public float moveSpeed;
 // Use this for initialization
 void Start () {
 
     //player = GameObject.FindGameObjectWithTag ("Player");
     player = GameObject.FindGameObjectWithTag ("Player").GetComponent<Player>();
     anim = gameObject.GetComponent<Animator> ();

     currHealth = maxHealth;
     anim.SetBool ("Angry", false);
 }
 
 // Update is called once per frame
 void Update () {
 
     //movement
     //transform.Translate (new Vector3 (0, moveSpeed, 0) * Time.deltaTime);

     if (currHealth <= rageLimit) {
         Angry ();
     }
     if (currHealth <= 0) {
         Die ();
     }
 }

// IEnumerator FireHands(){ // return yield 0; // } // // IEnumerator FireProjectiles(){ // return yield 0; // }

 public void Damage(int dmg){
     currHealth -= dmg;
 }

 void Die(){
     Destroy (gameObject);
 }

 void OnCollisionEnter2D(Collision2D col){
     if (col.collider.CompareTag("Bound")){
         moveSpeed *= -1;
     }
 }

 void Angry(){
     anim.SetBool ("Angry", true);
     transform.Translate (new Vector3 (0, moveSpeed, 0) * Time.deltaTime);
 }

}