NullReferenceException?

Question

Unity

NullReferenceException: Object reference not set to an instance of an object
PickUp.OnTriggerEnter2D (UnityEngine.Collider2D other) (at Assets/Scripts/Inventory/PickUp.cs:25)

public class PickUp : MonoBehaviour
{

    private Inventory inventory;
    public GameObject slotButton;

    void Start()
    {
        inventory = GameObject.FindGameObjectWithTag("Player").GetComponent<Inventory>();
    }

    private void OnTriggerEnter2D(Collider2D other)
    {
        if (other.CompareTag("Player"))
        {
            for (int i = 0; i < inventory.slots.Length ; i++)
            {
                if (inventory.isFull[i] == false)
                {
                    inventory.isFull[i] = true;
                    Instantiate(slotButton, inventory.slots[i].transform);
                    Destroy(gameObject);
                    break;
                }
            }
        }
    }
}

Sorry for the stupidity, maybe, but I just started working with the unit and this error is incomprehensible to me (Please help.