whats wrong....

NullReferenceException: Object reference not set to an instance of an object Done_DestroyByContact.OnTriggerEnter (UnityEngine.Collider other) (at Assets/Done/Done_Scripts/Done_DestroyByContact.cs:42)

Im fallowing along with the space shooter project and i was able to get everything to work. I then brought in my own models and for some reason im getting an error and my objects are not doing what they are suppose to

 using UnityEngine;
 using System.Collections;
 
 public class Done_DestroyByContact : MonoBehaviour
 {
     public GameObject explosion;
     public GameObject playerExplosion;
     public int scoreValue;
     private Done_GameController gameController;
 
     void Start ()
     {
         GameObject gameControllerObject = GameObject.FindGameObjectWithTag ("GameController");
         if (gameControllerObject != null)
         {
             gameController = gameControllerObject.GetComponent <Done_GameController>();
         }
         if (gameController == null)
         {
             Debug.Log ("Cannot find 'GameController' script");
         }
     }
 
     void OnTriggerEnter (Collider other)
     {
         if (other.tag == "Boundary" || other.tag == "Enemy")
         {
             return;
         }
 
         if (explosion != null)
         {
             Instantiate(explosion, transform.position, transform.rotation);
         }
 
         if (other.tag == "Player")
         {
             Instantiate(playerExplosion, other.transform.position, other.transform.rotation);
             gameController.GameOver();
         }
         
         gameController.AddScore(scoreValue);
         Destroy (other.gameObject);
         Destroy (gameObject);
     }
 }