Why is the field null if I assign it?

Question

Unity

Like there is this:

public class InventoryCell : MonoBehaviour, IPointerClickHandler
{
    [SerializeField] private TMP_Text _amountText;
    [SerializeField] private Image _iconField;

    [HideInInspector] public UIInventory _uiInventory;
    [HideInInspector] public Item _item;

    public void OnPointerClick(PointerEventData eventData)
    {
        print(_uiInventory);
        _uiInventory.RenderDescription(_item); // равно null
    }
}

public class UIInventory : MonoBehaviour
{

        [SerializeField] private InventoryCell _inventotyCell;

        Inventory.Items.ForEach(item =>
        {
            var Amount = item.Amount;

            while(true)
            {
                if(Amount >= item.MaxAmount)
                {
                    var cell = Instantiate(_inventotyCell, _contaner);
                    cell._item = item;
                    cell._uiInventory = this;
                    cell.Render(item.MaxAmount, item.Image);

                    Amount -= item.MaxAmount;
                }
                else
                {
                    var cell = Instantiate(_inventotyCell, _contaner);
                    cell._item = item;
                    cell.Render(Amount, item.Image);

                    Amount = 0;
                }

                if(Amount == 0)
                {
                    break;
                }
            }
        });
}