Items not saving because of NullReferenceException

Unity Engine
1

Hi! I might have to apologize for the dumb question but I don’t really understand why I get this error. Every time I load a shop in my game it gives me an error:

NullReferenceException: Object reference not set to an instance of an object
ShopController.IsItemBought (SpellPattern spell) (at Assets/Scripts/Unity/ShopController.cs:13)
ShopItem.Start () (at Assets/Scripts/Unity/ShopItem.cs:43)

here’s ShopController.cs

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

public class ShopController : PSingleton<ShopController>
{
    private List<SpellPattern> m_boughtSpells = new List<SpellPattern>();

    public bool IsItemBought(SpellPattern spell)
    {
        foreach(var el in m_boughtSpells)
        {
            if (el.Equals(spell))
            {
                return true;
            }
        }

        return false;
    }

    public void AddNewBoughtItem(SpellPattern item)
    {
        if (!m_boughtSpells.Contains(item))
        {
            m_boughtSpells.Add(item);
            GameManager.Instance.SaveGame();
            GameManager.Instance.LoadGame();
        }
    }

    protected override void Load()
    {
        m_boughtSpells = gameData.boughtSpells;
    }

    protected override void Save()
    {
        gameData.boughtSpells = m_boughtSpells;
    }
}

and here’s start void from ShopItem.cs

  private void Start()
    {
        anim = tutorialPointer.GetComponent<Animation>();

        isBought = ShopController.Instance.IsItemBought(spellPattern);
    }
2

Looks like the element you are getting from the “foreach” is null. Which probably is because either you are adding a null element in “AddNewBoughtItem” or your saving/loading system is messing with it.

3

Yep. You really should be checking for null items in the AddNewBoughtItem, Save and Load methods to catch any nulls slipping into the collection as soon as possible.

public void AddNewBoughtItem([NotNull]SpellPattern item)
{
    if(item == null)
    {
        throw new ArgumentNullException("AddNewBoughtItem called with a null item!");
    }
    ...
}

protected override void Load()
{
    m_boughtSpells = gameData.boughtSpells.RemoveNullValues("Bought Spells contained a null item after loading.");
}

protected override void Save()
{
    gameData.boughtSpells = m_boughtSpells.RemoveNullValues("Bought Spells contained a null item when saving.");
}

public static class ListExtensions
{
    public static void RemoveNullValues<T>([NotNull]this List<T> list, string errorIfNullFound) where T : class
    {
        for(int n = list.Count - 1; n >= 0; n--)
        {
            if(list[n] == null)
            {
                Debug.LogError(errorIfNullFound);
                list.RemoveAt(n);
            }
        }
    }
}