Dialog issue

I Have a problem with a dialog script I have been working on when you press the continue button in the User interface it doesn't display the next sentence however there isn't any errors

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using TMPro;
 
 public class Dialog : MonoBehaviour {
 
     public GameObject DialogUI;
 
     private Gamemaster gm;
 
     public TextMeshProUGUI testDisplay;
     public string[] sentences;
     public float typingSpeed = 0.02f;
     private int index;
 
      void OnTriggerEnter(Collider col)
     {
         if (col.CompareTag("Player"))
         {
             gm.HelpText.text = ("Press [E] To Talk");
             if (Input.GetKey("e"))
             {
 
                 Time.timeScale = 0f;
                 gm.HelpText.text = (" ");
                 DialogUI.SetActive(true);
             }
         }
     }
 
      void OnTriggerStay(Collider col)
     {
         if (col.CompareTag("Player"))
         {
             gm.HelpText.text = ("Press [E] To Talk");
             if (Input.GetKey("e"))
             {
                 gm.HelpText.text = (" ");
                 DialogUI.SetActive(true);
             }
         }
     }
 
     void Start()
     {
         gm = GameObject.FindGameObjectWithTag("GameMaster").GetComponent<Gamemaster>();
         StartCoroutine(Type());
     }
 
     IEnumerator Type()
     {
         foreach(char letter in sentences[index].ToCharArray())
         {
             testDisplay.text += letter;
             yield return new WaitForSeconds(typingSpeed);
 
         }
     }
 
     public void NextSentence()
     {
         if(index < sentences.Length -1){
             index++;
             testDisplay.text = "";
             StopCoroutine(Type());
         } else {
             testDisplay.text = "";
 
         }
     }
 
     public void ExitDialog()
     {
         Time.timeScale = 1f;
         DialogUI.SetActive(false);
     }
 
 }