NullReferenceException Error

Unity Engine
,
1

Hello I have this error, what did I do wrong?

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystems;

public class ItemDragAndDropController : MonoBehaviour
{

    [SerializeField] ItemSlot itemSlot;
    [SerializeField] GameObject itemIcon;
    RectTransform iconTransform;
    Image itemIconImage;

    private void Start()
    {
        itemSlot = new ItemSlot();
        iconTransform = itemIcon.GetComponent<RectTransform>();
        itemIconImage = itemIcon.GetComponent<Image>();
    }

    private void Update()
    {
        if (itemIcon.activeInHierarchy == true)
        {
            iconTransform.position = Input.mousePosition;

            if (Input.GetMouseButtonDown(0))
            {
                if (EventSystem.current.IsPointerOverGameObject() == false)
                {
                    Vector3 worldPosition = Camera.main.ScreenToWorldPoint(Input.mousePosition);
                    worldPosition.z = 0;

                    ItemSpawnManager.instance.SpawnItem(
                        worldPosition,
                        itemSlot.item,
                        itemSlot.count
                        );
                }
            }
        }
    }

    internal void OnClick(ItemSlot itemSlot)
    {
        if (this.itemSlot.item == null)
        {
            this.itemSlot.Copy(itemSlot);
            itemSlot.Clear();
        }
        else{
            Item item = itemSlot.item;
            int count = itemSlot.count;

            itemSlot.Copy(this.itemSlot);
            this.itemSlot.Set(item, count);
        }
        UpdateIcon();
    }

    private void UpdateIcon()
    {
        if (itemSlot.item == null)
        {
            itemIcon.SetActive(false);
        }
        else {
            itemIcon.SetActive(true);
            itemIconImage.sprite = itemSlot.item.icon;
        }
    }

}
2

ItemSpawnManager.instance don’t exist aka is null.

3

I have another file here

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ItemSpawnManager : MonoBehaviour
{
    public static ItemSpawnManager instance;

    private void Awake()
    {
        instance = this;
    }

    [SerializeField] GameObject pickUpItemPrefab;

    public void SpawnItem(Vector3 position, Item item, int count)
    {
        GameObject o = Instantiate(pickUpItemPrefab, position, Quaternion.identity);
        o.GetComponent<PickUpItem>().Set(item, count);
    }
}
4

About above script you drag and drop it to some object in hierarchy? So it can use Awake method.

5

So at the moment I only get this NullReferenceException error when I throw something on the ground from my inventory in the game, I don’t get any other errors.

6

I asked if you drag and drop ItemSpawnManager on any game object in hierarchy. If not its instance will be always null and result will be error like you posted.

7

The answer is always the same… ALWAYS!

How to fix a NullReferenceException error

https://forum.unity.com/threads/how-to-fix-a-nullreferenceexception-error.1230297/

Three steps to success:

  • Identify what is null ← any other action taken before this step is WASTED TIME
  • Identify why it is null
  • Fix that
8

I fixed the problem, thank you.