Coroutine variable is glitchy

Unity Engine
,
1

in this script, it heals me when i am less than 100% health but when i am at 100% health, it gives me error message saying that coroutine is null, help!

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

public class HealingCircle : MonoBehaviour
{
    public PlayerStats pS;
    public int healthPerSecond;

    Coroutine currentHealRoutine = null;

    void OnTriggerEnter2D(Collider2D collision)
    {
        if (collision.gameObject.CompareTag("Player") && pS.currentHealth < pS.maxHealth)
        {
            currentHealRoutine = StartCoroutine(Heal());
        }
    }

    void OnTriggerExit2D(Collider2D collision)
    {
        if (collision.gameObject.CompareTag("Player"))
        {
            StopCoroutine(currentHealRoutine);
        }
    }

    IEnumerator Heal()
    {
        for (int _currentHealth = pS.currentHealth; _currentHealth < pS.maxHealth; _currentHealth += healthPerSecond)
        {
            if ((pS.currentHealth = _currentHealth) > pS.maxHealth) //Check if the current health is more than max health
            {
                pS.currentHealth = pS.maxHealth; //If yes, keep current health as max health
                Debug.Log(pS.currentHealth);
            }
            else
            {
                pS.currentHealth = _currentHealth;
                Debug.Log(pS.currentHealth);
                yield return new WaitForSeconds(1f); //Wait for 1 second
            }
        }
        pS.currentHealth = pS.maxHealth; //Set the current health to max health.
    }

}
2 
    void OnTriggerEnter2D(Collider2D collision)
    {
        if (collision.gameObject.CompareTag("Player") && pS.currentHealth < pS.maxHealth)
        {
            currentHealRoutine = StartCoroutine(Heal());
        }
    }

Your OnTriggerEnter is checking if currentHealth is less than maxHealth. If it is, the coroutine is never started/created. Therefore it will be null when you exit. Should be simple to fix this just by checking if the coroutine is null before trying to stop it.

3

how can i prevent this?