Код выдает NullReferenceException

Код выдает NullReferenceException и я не могу понять почему, ругается на

HealthBarController.RefreshHealthBar(currentHealth, maxHealth);

PlayerController.cs

[Header ("Health")]
[SerializeField] public HealthBarController healthBarController;
[SerializeField] private float maxHealth;
[SerializeField] private float currentHealth;

 public void HealingPotionUsing()
 {
     if (healingPotionAmount != 0)
     {
         if (currentHealth == maxHealth)
         {
             currentHealth += 0;
         }
         else
         {
             currentHealth += healingAmount;
             HealthBarController.RefreshHealthBar(currentHealth, maxHealth); //налл референс эксепт???
             healingPotionAmount -= 1;
             PlayerUIController.RefreshHealingPotionText(healingPotionAmount);
         }
     }
 }

HealthBarController.cs

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

public class HealthBarController : MonoBehaviour
{
    [SerializeField] private float animTime;
    [SerializeField] private Image healthBar;
    private float elapsedTime;

    public void RefreshHealthBar(float currentHealth, float maxHealth)
    {
        healthBar.fillAmount = currentHealth / maxHealth;
        StartCoroutine(HealthbarAnim(currentHealth, currentHealth / maxHealth));
    }


    private IEnumerator HealthbarAnim(float startPos, float endPos)
    {
        while(elapsedTime < animTime)
        {
            healthBar.fillAmount = Mathf.Lerp(startPos, endPos, (elapsedTime / animTime));
            elapsedTime += Time.deltaTime;
            yield return null;
        }
    }
}