How to fix dialog system looping error 2D unity?

I'm following a tutorial on youtube(https://www.youtube.com/watch?v=1nFNOyCalzo) on how to make a simple dialogue system that when you collide on the NPC and press E a dialogue box with dialog and a continue button will appear, I changed it a bit; instead of pressing E, I try assigning it on a button because I'm making a mobile game so I make a public void InteractButton(). the problem is instead of displaying the line or sentence it loops and blinks like a Christmas light and it does not show the text properly, if I put Help it shows elpH. what should I do?

here's the code that I put on NPC

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

public class DialogueManager : MonoBehaviour {

 public string[] dialogue;
 private int index;
 public GameObject dialoguePanel;
 public TextMeshProUGUI dialogueText;
 public GameObject continueButton;
 public float typingSpeed;
 public bool playerIsClose;
 public bool isClick;

 // Update is called once per frame
 void Update()
 {
     if (isClick && playerIsClose) 
     {
         if (dialoguePanel.activeInHierarchy) 
         {
             zeroText();
         }
         else 
         {
             dialoguePanel.SetActive(true);
             StartCoroutine(Type());
         }
     }

     if (dialogueText.text == dialogue[index])
     {
         continueButton.SetActive(true);
     }
 }

 public void zeroText() 
 {
     dialogueText.text = "";
     index = 0;
     dialoguePanel.SetActive(false);
 }

 IEnumerator Type()
 {
     foreach (char letter in dialogue[index].ToCharArray())
     {
         dialogueText.text += letter;
         yield return new WaitForSeconds(typingSpeed);
     }
 }

 public void NextSentence()
 {
     continueButton.SetActive(false);
     if (index < dialogue.Length - 1)
     {
         index++;
         dialogueText.text = "";
         StartCoroutine(Type());
     }
     else
     {
         zeroText();
     }
 }

 public void InteractButton() 
 {
     isClick = true;
 }

 private void OnTriggerEnter2D(Collider2D other)
 {
     if (other.CompareTag("Player")) 
     {
         playerIsClose = true;
     }
 }

 private void OnTriggerExit2D(Collider2D other)
 {
     if (other.CompareTag("Player"))
     {
         playerIsClose = false;
         zeroText();
     }
 }

}