NullReferenceException Error

I'm creating a simple child app in Unity. The function that I am making now is a shop function.
When child buys items at the shop, I want to make sure that child can't buy them again.
Once an item is purchased at a shop, the item's information is stored in the Inventory.
IsInven() is to see if there are any items that child wants to purchase in the Inventory already, and if there are items in the Inventory that child wants to purchase, attempt to disable them.
However, a Null Reference Exception error occurred. I want to check which part is wrong.

The following is the content of the error.

Code (CSharp):

1.  
2. using System.Collections;
3. using System.Collections.Generic;
4. using UnityEngine;
5. using UnityEngine.UI;
6. using System;
7. public class Store_Sky : MonoBehaviour
8. {
9.     public ItemBuffer itemBuffer;
10.     public Transform slotRoot;
11.     public Action<ItemProperty> onClickBuy;
12.     private List<Slot> slots;
13.     public Inventory inven;
14.     //게임데이터 파일 만들면 삭제하기
15.     int userStar = 4;
16.     void FixedUpdate()
17.     {
18.         slots = new List<Slot>();
19.         int slotCnt = slotRoot.childCount;
20.         for(int i = 0; i < slotCnt; i++)
21.         {
22.             var slot = slotRoot.GetChild(i).GetComponent<Slot>();
23.             if(slot.item.starNum <= userStar)
24.             {
25.                 slot.SetItem(itemBuffer.items[i+21]);
26.             }
27.             else
28.             {
29.                 slot.GetComponent<Button>().interactable = false;
30.                 onClickBuy = null;
31.             }
32.             slots.Add(slot);
33.         }
34.     }
35.     public void OnclickBuy(Slot slot)
36.     {
37.         if (userStar >= 0 && IsInven(slot) == false)
38.         {
39.             onClickBuy?.Invoke(slot.item);
40.             userStar -= slot.item.starNum;
41.             Debug.Log(userStar);
42.         }
43.     }
44.     private bool IsInven(Slot slot)
45.     {
46.         var invenList = inven.slots.FindAll(t =>
47.         {
48.             return t.item != null || t.item.itemname != string.Empty;
49.         });
50.         if(invenList != null)
51.         {
52.             var isInven = invenList.Find(p =>
53.             {
54.                 return p.item.itemID == slot.item.itemID;
55.             });
56.             return isInven != null || isInven.item.itemname != string.Empty;
57.         }
58.         else
59.         {
60.             return false;
61.         }
62.     }
63. }
64.  
65. }

 

Code (CSharp):

1. return isInven != null || isInven.item.itemname != string.Empty;

I think you want to use && instead of || here