How to fix NullReferenceException: Object reference not set to an instance of an object error in Unity

Question

I am working on an inventory and every time I opne it I get this error: NullReferenceException: Object reference not set to an instance of an object Inventory.LoadUnloadInventoryMenu () (at Assets/Scripts/Inventory.cs:116) Inventory.Update () (at Assets/Scripts/Inventory.cs:59)

This is the inventory.cs script:

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

public class Inventory : MonoBehaviour
{
    //Keys used to open doors
    public bool hasRedKey, hasGreenKey, hasBlueKey;

    //the item the player is using
    public Sprite mainItem;

    public Sprite[] hotbar = new Sprite[3];


    //every item in the game
    public Sprite[] items = new Sprite[30];


    //items in the players inventory
    public Sprite[] inventory = new Sprite[20];

    //armor slots in thye players Inventory
    public Sprite[] armor = new Sprite[4];

    //the inventory menu
    public GameObject menu;

    void Start()
    {
        //Deactivating the menu
        menu.SetActive(false);

        //assinging values
        mainItem = items[0];
        FindObjectOfType<CanvasManager>().UpdateMainItem(mainItem);
        hotbar[0] = items[3];
        hotbar[1] = items[20];
        hotbar[2] = items[4];
        FindObjectOfType<CanvasManager>().UpdateHotbar(hotbar);

        for(int i = 0; i < 20; i++)
        {
            inventory[i] = items[i];
        }

        for (int i = 0; i < 4; i++)
        {
            armor[i] = items[i];
        }
    }

    void Update()
    {
        //checking if the player opened/ closed the inventory
        if (Input.GetKeyDown(KeyCode.E))
        {
            LoadUnloadInventoryMenu();
        }


        //cycle through hotbar
        if (Input.mouseScrollDelta.y == 1)
        {
            nextItem();
        }

        if (Input.mouseScrollDelta.y == -1)
        {
            prevItem();
        }
    }

    private void nextItem()
    {
        Sprite temp = hotbar[0];
        hotbar[0] = hotbar[1];
        hotbar[1] = hotbar[2];
        hotbar[2] = mainItem;
        mainItem = temp;
        FindObjectOfType<CanvasManager>().UpdateHotbar(hotbar);
        FindObjectOfType<CanvasManager>().UpdateMainItem(mainItem);

    }

    private void prevItem()
    {
        Sprite temp = mainItem;
        mainItem = hotbar[2];
        hotbar[2] = hotbar[1];
        hotbar[1] = hotbar[0];
        hotbar[0] = temp;
        FindObjectOfType<CanvasManager>().UpdateHotbar(hotbar);
        FindObjectOfType<CanvasManager>().UpdateMainItem(mainItem);
    }


    private void LoadUnloadInventoryMenu()
    {
        if (menu.activeSelf)
        {
            //diactivatting the inventory when its open
            menu.SetActive(false);
        }
        else
        {
            //activatting the inventory
            menu.SetActive(true);

            //null check
            if(inventory != null && armor != null && hotbar != null)
            {

                //update the items to the correct value
                GameObject.Find("InventoryManager").GetComponent<InventoryManager>().UpdateInventory();
                GameObject.Find("InventoryManager").GetComponent<InventoryManager>().UpdateHotbar();
                GameObject.Find("InventoryManager").GetComponent<InventoryManager>().UpdateArmor();

            }
                
        }
            
    }
}

The Canvas manager is used to display the UI visible throughout the game that displays things like health to the player. The "menu" object I assigned manually in the Unity Inspector as the InventoryManager object which is the InventoryUI. This is the InventoryManager.cs file:

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

public class InventoryManager : MonoBehaviour
{
    //definning inventory slots
    public Image[] inventory = new Image[20];
    public Image[] armor = new Image[4];
    public Image[] hotbar = new Image[3];


    //setting the hotbar slots to the correct values
    public void UpdateHotbar()
    {
        Sprite[] toLoad = GameObject.Find("Inventory").GetComponent<Inventory>().hotbar;

        for(int i = 0; i < hotbar.Length; i++)
        {
            hotbar[i].sprite = toLoad[i];
        }
    }

    //setting the armor slots to the correct values
    public void UpdateArmor()
    {
        Sprite[] toLoad = GameObject.Find("Inventory").GetComponent<Inventory>().armor;

        for(int i = 0;i < armor.Length; i++)
        {
            armor[i].sprite = toLoad[0];
        }
    }

    //setting the inventory slots to the correct values
    public void UpdateInventory()
    {
        Sprite[] toLoad = GameObject.Find("Inventory").GetComponent<Inventory>().inventory;

        for (int i = 0; i < inventory.Length; i++)
        {
            inventory[i].sprite = toLoad[0];
        }
    }
}

If anyone know when I'm getting this error please let me know. Thanks in advance.