Porque me pone esto?

Tengo un script del asset Inventory Master lo he modificado para el juego que estoy desarrollando. Es el siguiente:

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

public class PlayerInventory : MonoBehaviour
{
    public GameObject inventory;
    public GameObject characterSystem;
    public GameObject craftSystem;
    private Inventory craftSystemInventory;
    private CraftSystem cS;
    private Inventory mainInventory;
    private Inventory characterSystemInventory;
    private Tooltip toolTip;

private InputManager inputManagerDatabase;

public GameObject HPMANACanvas;

public Text hpText;
public Text hambreText;
public Text sedText;
public Image hpImage;
public Image hambreImage;
public Image sedImage;

float maxHealth = 100;
float maxHambre = 100;
float maxSed = 100;


public float currentHealth = 100;
public float currentHambre = 100;
public float currentSed = 100;


int normalSize = 3;

public void OnEnable()
{
    Inventory.ItemEquip += OnBackpack;
    Inventory.UnEquipItem += UnEquipBackpack;

    Inventory.ItemEquip += OnGearItem;
    Inventory.ItemConsumed += OnConsumeItem;
    Inventory.UnEquipItem += OnUnEquipItem;

    Inventory.ItemEquip += EquipWeapon;
    Inventory.UnEquipItem += UnEquipWeapon;
}

public void OnDisable()
{
    Inventory.ItemEquip -= OnBackpack;
    Inventory.UnEquipItem -= UnEquipBackpack;

    Inventory.ItemEquip -= OnGearItem;
    Inventory.ItemConsumed -= OnConsumeItem;
    Inventory.UnEquipItem -= OnUnEquipItem;

    Inventory.UnEquipItem -= UnEquipWeapon;
    Inventory.ItemEquip -= EquipWeapon;
}

void EquipWeapon(Item item)
{
    if (item.itemType == ItemType.Weapon)
    {
        //add the weapon if you unequip the weapon
    }
}

void UnEquipWeapon(Item item)
{
    if (item.itemType == ItemType.Weapon)
    {
        //delete the weapon if you unequip the weapon
    }
}

void OnBackpack(Item item)
{
    if (item.itemType == ItemType.Backpack)
    {
        for (int i = 0; i < item.itemAttributes.Count; i++)
        {
            if (mainInventory == null)
                mainInventory = inventory.GetComponent<Inventory>();
            mainInventory.sortItems();
            if (item.itemAttributes[i].attributeName == "Slots")
                changeInventorySize(item.itemAttributes[i].attributeValue);
        }
    }
}

void UnEquipBackpack(Item item)
{
    if (item.itemType == ItemType.Backpack)
        changeInventorySize(normalSize);
}

void changeInventorySize(int size)
{
    dropTheRestItems(size);

    if (mainInventory == null)
        mainInventory = inventory.GetComponent<Inventory>();
    if (size == 3)
    {
        mainInventory.width = 3;
        mainInventory.height = 1;
        mainInventory.updateSlotAmount();
        mainInventory.adjustInventorySize();
    }
    if (size == 6)
    {
        mainInventory.width = 3;
        mainInventory.height = 2;
        mainInventory.updateSlotAmount();
        mainInventory.adjustInventorySize();
    }
    else if (size == 12)
    {
        mainInventory.width = 4;
        mainInventory.height = 3;
        mainInventory.updateSlotAmount();
        mainInventory.adjustInventorySize();
    }
    else if (size == 16)
    {
        mainInventory.width = 4;
        mainInventory.height = 4;
        mainInventory.updateSlotAmount();
        mainInventory.adjustInventorySize();
    }
    else if (size == 24)
    {
        mainInventory.width = 6;
        mainInventory.height = 4;
        mainInventory.updateSlotAmount();
        mainInventory.adjustInventorySize();
    }
}

void dropTheRestItems(int size)
{
    if (size < mainInventory.ItemsInInventory.Count)
    {
        for (int i = size; i < mainInventory.ItemsInInventory.Count; i++)
        {
            GameObject dropItem = (GameObject)Instantiate(mainInventory.ItemsInInventory[i].itemModel);
            dropItem.AddComponent<PickUpItem>();
            dropItem.GetComponent<PickUpItem>().item = mainInventory.ItemsInInventory[i];
            dropItem.transform.localPosition = GameObject.FindGameObjectWithTag("Player").transform.localPosition;
        }
    }
}

void Start()
{
    if (HPMANACanvas != null)
    {
        hpText = HPMANACanvas.transform.GetChild(1).GetChild(0).GetComponent<Text>();

        hambreText = HPMANACanvas.transform.GetChild(2).GetChild(0).GetComponent<Text>();

        sedText = HPMANACanvas.transform.GetChild(3).GetChild(0).GetComponent<Text>();

        hpImage = HPMANACanvas.transform.GetChild(1).GetComponent<Image>();
        hambreImage = HPMANACanvas.transform.GetChild(1).GetComponent<Image>();
        sedImage = HPMANACanvas.transform.GetChild(1).GetComponent<Image>();

        UpdateSedBar();
        UpdateHPBar();
        UpdateHambreBar();
    }

    if (inputManagerDatabase == null)
        inputManagerDatabase = (InputManager)Resources.Load("InputManager");

    if (craftSystem != null)
        cS = craftSystem.GetComponent<CraftSystem>();

    if (GameObject.FindGameObjectWithTag("Tooltip") != null)
        toolTip = GameObject.FindGameObjectWithTag("Tooltip").GetComponent<Tooltip>();
    if (inventory != null)
        mainInventory = inventory.GetComponent<Inventory>();
    if (characterSystem != null)
        characterSystemInventory = characterSystem.GetComponent<Inventory>();
    if (craftSystem != null)
        craftSystemInventory = craftSystem.GetComponent<Inventory>();
}

void UpdateHPBar()
{
   hpText.text = (currentHealth + "/" + maxHealth);
    float fillAmount = currentHealth / maxHealth;
   hpImage.fillAmount = fillAmount;
}

void UpdateHambreBar()
{
    hambreText.text = (currentHambre + "/" + maxHambre);
    float fillAmount = currentHambre / maxHambre;
    hambreImage.fillAmount = fillAmount;
}
void UpdateSedBar()
{
    sedText.text = (currentSed + "/" + maxSed);
    float fillAmount = currentSed / maxSed;
    sedImage.fillAmount = fillAmount;
}



public void OnConsumeItem(Item item)
{
    for (int i = 0; i < item.itemAttributes.Count; i++)
    {
        if (item.itemAttributes[i].attributeName == "Health")
        {
            if ((currentHealth + item.itemAttributes[i].attributeValue) > maxHealth)
                currentHealth = maxHealth;
            else
                currentHealth += item.itemAttributes[i].attributeValue;
        }
        if (item.itemAttributes[i].attributeName == "Hambre")
        {
            if ((currentHambre + item.itemAttributes[i].attributeValue) > maxHambre)
                currentHambre = maxHambre;
            else
                currentHambre += item.itemAttributes[i].attributeValue;
        }
        if (item.itemAttributes[i].attributeName == "Sed")
        {
            if ((currentSed + item.itemAttributes[i].attributeValue) > maxSed)
                currentSed = maxSed;
            else
                currentSed += item.itemAttributes[i].attributeValue;
        }
       
    }
    if (HPMANACanvas != null)
    {
       UpdateSedBar();
        UpdateHPBar();
        UpdateHambreBar();
    }
}

public void OnGearItem(Item item)
{
    for (int i = 0; i < item.itemAttributes.Count; i++)
    {
        if (item.itemAttributes[i].attributeName == "Health")
            maxHealth += item.itemAttributes[i].attributeValue;
        if (item.itemAttributes[i].attributeName == "Hambre")
            maxHambre += item.itemAttributes[i].attributeValue;
        if (item.itemAttributes[i].attributeName == "Sed")
            maxSed += item.itemAttributes[i].attributeValue;
        
    }
    if (HPMANACanvas != null)
    {
        UpdateSedBar();
        UpdateHPBar();
        UpdateHambreBar();
    }
}

public void OnUnEquipItem(Item item)
{
    for (int i = 0; i < item.itemAttributes.Count; i++)
    {
        if (item.itemAttributes[i].attributeName == "Health")
            maxHealth -= item.itemAttributes[i].attributeValue;
        if (item.itemAttributes[i].attributeName == "Hambre")
            maxHambre -= item.itemAttributes[i].attributeValue;
        if (item.itemAttributes[i].attributeName == "Sed")
            maxSed -= item.itemAttributes[i].attributeValue;
 
    }
    if (HPMANACanvas != null)
    {
        UpdateHambreBar();
        UpdateHPBar();
        UpdateSedBar();
    }
}

Y cuando le doy al play la consola me dice:
NullReferenceException: Object reference not set to an instance of an object
PlayerInventory.UpdateSedBar () (at Assets/InventoryMaster/Scripts/ExampleScript/PlayerInventory.cs:210)
PlayerInventory.Start () (at Assets/InventoryMaster/Scripts/ExampleScript/PlayerInventory.cs:174)

Alguna idea de porque pasa esto?