adding inventory items is half-working

Hello all, I'm really struggling with this one, please help me. First, let me show you the code.`

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 using System.Collections.Generic;
 
 public class Inventory : MonoBehaviour 
 {
     GameObject inventoryPanel;
     GameObject slotPanel;
     itemDataBase database;
     public GameObject inventorySlot;
     public GameObject inventoryItem;
 
     int slotAmount;
     public List<Item> items = new List<Item>();
     public List<GameObject> slots = new List<GameObject>();
 
     void Start()
     {
         database = GetComponent<itemDataBase> ();
 
         slotAmount = 36;
         inventoryPanel = GameObject.Find ("Inventory Panel");
         slotPanel = inventoryPanel.transform.FindChild ("Slot Panel").gameObject;
         for (int i = 0; i < slotAmount; i++) 
         {
             items.Add (new Item ());
             slots.Add (Instantiate (inventorySlot));
             slots [i].transform.SetParent (slotPanel.transform);
         }
 
         AddItem (5);
     }
 
     public void AddItem(int id)
     {
         Item itemToAdd = database.FetchItemByID (id);
         for (int i = 0; i < items.Count; i++) 
         {
             if (items [i].ID == -1) 
             {
                 items [i] = itemToAdd;
                 GameObject itemObj = Instantiate (inventoryItem);
                 itemObj.transform.SetParent (slots [i].transform);
                 itemObj.transform.position = new Vector2(0,0);
                 itemObj.GetComponent<Image>().sprite = itemToAdd.Sprite;
                 itemObj.name = itemToAdd.Title;
                 break;    
             }
         }
     }
 }
 

This is the Inventory class, as you can see, I called "AddItem" with the ID=5 in Start(). In the first image that I uploaded, you can see that the item indeed shows in my inventory. However, I don't want to call this function here. Let me paste the second code.

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 using System.Collections.Generic;
 
 public class axePickUp : MonoBehaviour {
     
     Inventory inventory;
 
     void Start () {
         inventory = GetComponent<Inventory> ();
     }
 
     void Update(){
         if (Input.GetButtonDown("F")) {
             pickUp();
         }
     }
 
     void pickUp(){
         inventory.AddItem (5);
     }
 }

I want to call this function in axePickUp(). The function kinda knows what I want, but not exactly, as you can see in the second image I uploaded....

The sprite image just sits there in the corner. PLEASE FOR THE LOVE OF GOD, help me. I have absolutely no idea how to fix this, I tried for days.