enemy projectiles

I am using an enemy projectile script that works fine until the player is killed. After the player is killed the red flag error "Null Reference Exception: Object reference not set to an instance of an object Enemy Star Controller .Start () (at Assets/Scripts/Enemy Star Controller.cs:28)" appears. The script is for a 2D platformer game. (more below)

Line 28 of the script is highlighted when I click on the error code however I do not know how to fix it and the code compiles without any problems. The Enemy Star Controller script is below any help will be greatly appreciated.

 1 using UnityEngine;
 2 using System.Collections;
 3
 4 public class EnemyStarController : MonoBehaviour {
 5.
 6         public float speed;
 7
 8        public PlayerController player;
 9
 10        //public GameObject enemyDeathEffect;
 11
 12        public GameObject impactEffect;
 13
 14        //public int points to kill;
 15
 16        public float rotationSpeed;
 17
 18        public int damageToGive;
 19
 20        private Rigidbody2D myrigidbody2D;
 21
 22        // Use this for initialization
 23        void Start () {
 24           player = FindObjectOfType<PlayerController> ();
 25    
 26           myrigidbody2D = GetComponent<Rigidbody2D> ();
 27    
 28           if (player.transform.position.x < transform.position.x)
 29            
 30           {
 31            speed = -speed;
 32            rotationSpeed = -rotationSpeed;
 33           }
 34           if (player.transform.position.x < transform.position.y)
 35          {
 36            speed = speed;
 37            rotationSpeed = rotationSpeed;
 38        }
 39   }
 40
 41    // Update is called once per frame
 42    void Update () {
 43        myrigidbody2D.velocity = new Vector2 (speed, myrigidbody2D.velocity.y);
 44
 45        myrigidbody2D.angularVelocity = rotationSpeed;
 46    }
 47
 48    void OnTriggerEnter2D(Collider2D other)
 49    {
 50        if(other.name == "Player")
 51        {
 52            //Instantiate(enemyDeathEffect,other.transform.position, other.transform.rotation);
 53            //Destroy (other.gameObject);
 54            //ScoreManager.AddPoints(pointsForKill);
 55
 56            //HealthManager.HurtPlayer (damageToGive);
 57        }
 58
 59        Instantiate (impactEffect, transform.position, transform.rotation);
 60        Destroy (gameObject);
 61        }
 62  }
 63