Unity - Destroying one duplicated enemy leads to can't attack anymore - unity3d

Question

I've created a "game" where you can attack enemies with a sword. All damages applies to only one enemy, even if i attack the other one. Also when I destroy the first I get this message
MissingReferenceException: The object of type 'RectTransform' has been destroyed but you are still trying to access it.
How do I fix it?
Here's my code:
Attached to the enemy
public class EnemyHealth : MonoBehaviour
{
private Transform healthBar;
private Transform canvas;
public float maxHealth = 100f;
public float damage = 10f;
float currentHealth;
float barLength;
private void Start()
{
currentHealth = maxHealth;
GameObject gfx = gameObject.transform.Find("GFX").gameObject;
canvas = gfx.transform.Find("Health Bar");
GameObject canv = canvas.gameObject;
healthBar = canv.transform.Find("Green Bar");
barLength = canvas.GetComponent<RectTransform>().rect.width * canvas.localScale.x;
}
public void GetStruck()
{
currentHealth -= damage;
if (currentHealth < 0) currentHealth = 0;
healthBar.localScale = new Vector3(currentHealth / maxHealth, 1f, 1f);
Vector3 p = healthBar.position;
p -= damage / (maxHealth * 2) * barLength * transform.right * -1;
healthBar.position = p;
if(currentHealth == 0)
{
Destroy(gameObject);
}
}
}
And the sword attack
using UnityEngine;
public class SwordAttack : MonoBehaviour
{
public Animator animator;
private EnemyHealth enemyHealth;
private bool isAttacking;
private void Start()
{
enemyHealth = GameObject.FindGameObjectWithTag("Enemy").GetComponent<EnemyHealth>();
}
void Update()
{
if(Input.GetMouseButtonDown(0))
{
isAttacking = true;
animator.Play("swordSwing", -1, 0f);
}
else if(animator.GetCurrentAnimatorStateInfo(0).normalizedTime > 1)
{
isAttacking = false;
}
animator.SetBool("isAttacking", isAttacking);
}
private void OnTriggerEnter(Collider other)
{
if(isAttacking && other.CompareTag("Enemy"))
{
enemyHealth.GetStruck();
}
}
}

Answer 1

private void Start()
{
enemyHealth = GameObject.FindGameObjectWithTag("Enemy").GetComponent<EnemyHealth>();
}
This is your mistake. You are only getting 1 enemy's health at your start method and you are always decreasing from that enemy's health. Instead you can get the hitted enemy's health in your OnTriggerEnter method and apply damage to that enemy.