NullReferenceException: Object reference not set to an instance of an object why?

Question

C++ / C#

Why does this error display?
Here is the code:

Inventory

for(int i = 0; i < inventoryItems.Length;i++)
        {

            if(inventoryItems[i] != null) continue;
            inventoryItems[i] = it;
            print("item add - " + inventoryItems[i]);
            (new InventoryWindow()).Redraw();
        }

InventoryWindow

InventoryWindow

public class InventoryWindow : MonoBehaviour
{
    [SerializeField] public Inventory targetInventory;
    [SerializeField] public Transform itemsPanel;
    void Start()
    {
        Redraw();
    }
    void Update() 
    {
        if(Input.GetKeyDown(KeyCode.F))
            Redraw();
    }
    public void Redraw()
    {
        bool ChildNull = true;
        bool RemoveSprites = false;
        bool finish = false;
        while(finish != true)
        {
            print("z - " + itemsPanel.transform.childCount);
            if(itemsPanel.transform.childCount == 0) {ChildNull = true;}
            else if(itemsPanel.transform.childCount > 0) {ChildNull = false;}

            print("bool - " + ChildNull);
            if(ChildNull == true)//если спрайтов нету 
            {
                for(var i = 0; i < targetInventory.inventoryItems.Length; i++)
                {
                    if(targetInventory.inventoryItems[i] != null)
                    {
                        var item = targetInventory.inventoryItems[i];

                        GameObject icon = new GameObject("Icon");
                        icon.AddComponent<Image>().sprite = item.Icon;
                        icon.transform.SetParent(itemsPanel);
                        icon.transform.localScale = Vector3.one;
                    }
                } 
            }else//если есть 
            {
                if(RemoveSprites == false)
                {
                    for(int i = 0; i < itemsPanel.transform.childCount; i++)   
                    {
                        Destroy(itemsPanel.transform.GetChild(i).gameObject);
                        if(i == (itemsPanel.transform.childCount - 1)) RemoveSprites = true;
                    }
                }else if(RemoveSprites == true)
                {
                    for(var i = 0; i < targetInventory.inventoryItems.Length; i++)
                    {
                        if(targetInventory.inventoryItems[i] != null)
                        {
                            var item = targetInventory.inventoryItems[i];

                            GameObject icon = new GameObject("Icon");
                            icon.AddComponent<Image>().sprite = item.Icon;
                            icon.transform.SetParent(itemsPanel);
                            icon.transform.localScale = Vector3.one;
                        }
                        if(i == (targetInventory.inventoryItems.Length - 1)) finish = true;
                    }    
                }
            }
        }
    }
}

Error on this line: , but if you remove this line, then this
print("z - " + itemsPanel.transform.childCount);

if(itemsPanel.transform.childCount == 0) {ChildNull = true;}

The error is shown if Redraw is called from Inventory, and if called from InventoryWindow, then this error is not present

Answer 1

Firstly, using While is bad.
Secondly, the NullReferenceException error means that you are trying to access an object that is not (null) or empty, you may have deleted it or it has not yet been created. From the described emerges that in itemsPanel.transform. childCount - null(empty). Alternatively, you can write print("z - " + itemsPanel.transform?.childCount);

Answer 2

(new InventoryWindow()).Redraw();

And it should also be written there that you do not need to nuke MonoBehaviour classes.
But if you do, make sure your SerializeField is set by you on your new instance.