i cant put text into a ui text thru a c# script

I have a problem using this code :

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class MouseController : MonoBehaviour {
     public GameObject Crosshair;
     private RaycastHit rayHit;
 
     public Text HitsTXT;
     public Text MissesTXT;
     public Text ShotsFiredTXT;
 
     public int hits;
     public int Misses;
     public int ShotsFired;
 
     // Use this for initialization
     void Start () {
         hits = 0;
         HitsTXT.text = "Hits : " + hits.ToString ();
 
         Misses = 0;
         MissesTXT.text = "Misses : " + Misses.ToString ();
 
         ShotsFired = 0;
         ShotsFiredTXT.text = "Shots Fired : " + ShotsFired.ToString ();
 
     }

it gives the following error :

NullReferenceException: Object reference not set to an instance of an object MouseController.Start () (at Assets/MouseController.cs:20)

The weird part is that the game pauses but it does change the text but i dont want the error/crash

How do i fix this?