NullReferenceException UnityEngine.Object.Instantiate Inventory UI Error

Questions & Answers
1

I am trying to make an inventory and line 30 is throwing an error when I play the game. Please help me understand how I can get it to work again because it was working until I made the ‘itemSlotContainer’ a prefab. Now that I remade it regular there’s still an issue. I also made sure to add the object in my game manager. Here is the error:

`NullReferenceException
UnityEngine.Object.Instantiate (UnityEngine.Object original, UnityEngine.Transform parent, Boolean instantiateInWorldSpace) (at C:/buildslave/unity/build/Runtime/Export/UnityEngineObject.cs:190)

UnityEngine.Object.Instantiate[Transform] (UnityEngine.Transform original, UnityEngine.Transform parent, Boolean worldPositionStays) (at C:/buildslave/unity/build/Runtime/Export/UnityEngineObject.cs:221)

UnityEngine.Object.Instantiate[Transform] (UnityEngine.Transform original, UnityEngine.Transform parent) (at C:/buildslave/unity/build/Runtime/Export/UnityEngineObject.cs:216)
UI_Inventory.RefreshInventoryItems () (at Assets/Scripts/UI_Inventory.cs:30)
UI_Inventory.SetInventory (.Inventory inventory) (at Assets/Scripts/UI_Inventory.cs:20)
GameManager.Awake () (at Assets/Scripts/GameManager.cs:31)
`

using UnityEngine;
using UnityEngine.UI;
public class UI_Inventory : MonoBehaviour {

    private Inventory inventory;
    private Transform itemSlotContainer;
    private Transform itemSlotTemplate;

    private void Awake()
    {
        itemSlotContainer = transform.Find("itemSlotContainer");
        itemSlotTemplate = itemSlotContainer.Find("itemSlotTemplate");


    }

    public void SetInventory(Inventory inventory)
    {
        this.inventory = inventory;
        RefreshInventoryItems();
    }

    private void RefreshInventoryItems()
    {
        int x = 0;
        int y = 0;
        float itemSlotCellSize = 70f;
        foreach (Item item in inventory.GetItemList())
        {
            RectTransform itemSlotRectTransform = Instantiate(itemSlotTemplate, itemSlotContainer).GetComponent<RectTransform>();
            itemSlotRectTransform.gameObject.SetActive(true);


            itemSlotRectTransform.anchoredPosition = new Vector2(x * itemSlotCellSize, y * itemSlotCellSize);
            Image image = itemSlotRectTransform.Find("Image").GetComponent<Image>();
            image.sprite = item.GetSprite();


            x++;

            if (x > 5)
            {
                x = 0;
                y++;
            }
        }
    }
}
2

It looks like you are calling RefreshInventoryItems from the Awake() function of you GameManager script. If the GameManager Awake function is called before the UI_Inventory Awake() function, then itemSlotContainer and itemSlotTemplate will be null. If RefresInventoryItems is called only once, I recommend just moving your Awake() code into that function so we can guarantee that these objects are being found before we try to use them. Otherwise, if that function is called more than once, perhaps you could add another function to your code, and have GameManager call the new function instead

public void InitializeInventoryItems () {
    itemSlotContainer = transform.Find ("itemSlotContainer");
    itemSlotTemplate = itemSlotContainer.Find ("itemSlotTemplate");
    RefreshInventoryItems ();
}