NullReferenceException: I need some help here

NullReferenceException: Object reference not set to an instance of an object that's what it says but I'm not sure how to fix it, I was able to figure out where its coming from tho. My ItemCounter script:

I'm kind of new to C# so I might have done something dumb.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class ItemCounter : MonoBehaviour {
 
     private bool finished = false;
     public int maxItems;
     public GameObject enableThis;
     public GameObject disableThis;
     public int currentItems = 0;
     
 
     void Update(){
         if(currentItems >= maxItems && finished == false){
             enableThis.SetActive(true);
             disableThis.SetActive(false);
             finished = true;
         }
     }
 
     void OnTriggerEnter(Collider items){
         if(items.gameObject.name == "item"){
             currentItems += 1;
         }
 
     }
 
     void OnTriggerExit(Collider leave){
         if (leave.gameObject.name == "item") {
             currentItems -= 1;
         }
     }
 
 }