Object reference not set to an instance of an object (c#)

The problem is on line 20 but as I have little knowledge on scripting i'm not sure what the problem is.

using UnityEngine; using System.Collections;

public class DestroyScript : MonoBehaviour

 {
 public GameObject Explosion;
 public int scoreValue;
 private GameController gameController;



 void OnTriggerEnter (Collider Other)
 {
     if(Other.tag == "Player")
     {
         Instantiate (Explosion,transform.position, transform.rotation);
         Destroy(gameObject);
         gameController.AddScore (scoreValue);
     }
 }

}