NullReferenceException in for-loop

So i'm still working on an doom-like fps and i wanted to implement some sort of gui to let the player know how many health is left and so on... but i simply won't work.

Here is the GUI Code: using UnityEngine; using System.Collections;

 public class PlayerStatsGUI : MonoBehaviour 
 {
     //Var initalization
     public Transform PlayerStatsObj;
     public StatsManager stats;
     public Shoot shoot;
 
     //Textures
     public Texture2D knifes;
 
     void Start()
     {
         stats = PlayerStatsObj.GetComponent<StatsManager>();
         shoot = PlayerStatsObj.GetComponent<Shoot>();
     }
 
     void OnGUI()
     {
         //Health
 
         //Ammo
         for(int i = 0; i < shoot.currentAmmo; i++)
         {
             GUI.DrawTexture(new Rect((Screen.width / 2) + (i * 100), (Screen.height / 2 - 400), 100,100), knifes);
         }
     }
 }

And this is the "ShootManagerScript":

 using UnityEngine;
 using System.Collections;
 
 public class Shoot : MonoBehaviour 
 {
     public int maxAmmo = 10;
     public int currentAmmo = 10;
 
     void Start()
     {
 
     }
 }
 

I'm getting an NullReferenceException Error at line 25 in my GUI Script...

Thanks for any help or advice :)