Trying to get Points to accumulate in game

I'm very new to Unity and I am trying to get a basic scoring system with shooting targets. I've got the score text working but it does not add any points. Each object is a basic cube with different point amounts added to each one and the Target script is attached. The Game Controller script is attached to the UI Text.

GameController.cs

using UnityEngine; using System.Collections; using UnityEngine.UI;

public class GameController : MonoBehaviour {

 Text text;
 private int score;

 // Use this for initialization
 void Start () {

     text = GetComponent <Text> ();

     score = 0;
     UpdateScore ();

 }

 public void AddScore (int newScoreValue)
 {
     score += newScoreValue;
     UpdateScore ();
         
 }

 void UpdateScore ()
 {
     text.text = "Score: " + score;    
 }
 // Update is called once per frame

}

Target.cs

using UnityEngine; using System.Collections;

public class Target : MonoBehaviour {

 public int scoreValue;
 private GameController gameController;

 // Use this for initialization
 void Start () { 
 }

 // Update is called once per frame
 void Update () {
 }

 void OnTriggerEnter(Collider ci)
 {
     Destroy (gameObject);
     Destroy (ci.gameObject);
     gameController.AddScore (scoreValue);

 }

}

When my bullet collides with the target I get an error in the console that says NullReferenceException: Object reference not set to an instance of an object Target.OnTriggerEnter (UnityEngine.Collider ci) (at Assets/Lab/Target.cs:31)