Ui Not Working [ Null Reference Exception ],Ui not Working [Null Reference Exception]

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1

using System.Collections;
using System.Collections.Generic;
using TMPro;
using UnityEngine;

public class UI : MonoBehaviour
{
[SerializeField] private TextMeshProUGUI healthText = default;
[SerializeField] private TextMeshProUGUI staminaText = default;

private void OnEnable()
{
    PlayerController.OnDamage += UpdateHealth;
    PlayerController.OnHeal += UpdateHealth;
    PlayerController.OnStaminaChange += UpdateStamina;

}
private void OnDisable()
{
  PlayerController.OnDamage -= UpdateHealth;
  PlayerController.OnHeal -= UpdateHealth;
  PlayerController.OnStaminaChange -= UpdateStamina;

}

private void Start()
{
    UpdateHealth(100);
    UpdateStamina(100);
}

private void UpdateHealth(float currentHealth)
{
    healthText.text = currentHealth.ToString("00");
}

private void UpdateStamina(float currentStamina) // THIS LINE RIGHT HERE
{
    staminaText.text =  currentStamina.ToString("00");
}

}
,using System.Collections;
using System.Collections.Generic;
using TMPro;
using UnityEngine;

public class UI : MonoBehaviour
{
[SerializeField] private TextMeshProUGUI healthText = default;
[SerializeField] private TextMeshProUGUI staminaText = default;

private void OnEnable()
{
    PlayerController.OnDamage += UpdateHealth;
    PlayerController.OnHeal += UpdateHealth;
    PlayerController.OnStaminaChange += UpdateStamina;

}
private void OnDisable()
{
  PlayerController.OnDamage -= UpdateHealth;
  PlayerController.OnHeal -= UpdateHealth;
  PlayerController.OnStaminaChange -= UpdateStamina;

}

private void Start()
{
    UpdateHealth(100);
    UpdateStamina(100);
}

private void UpdateHealth(float currentHealth)
{
    healthText.text = currentHealth.ToString("00");
}

private void UpdateStamina(float currentStamina) //THIS LINE RIGHT HERE
{
    staminaText.text =  currentStamina.ToString("00");
}

}