Analyzed from and to. What's wrong?

Question

C++ / C#

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

public class Dialogs : MonoBehaviour {
  
  public bool skipText;
  public bool isPrint;
  public static string textMess;
  public GameObject Textdia;


  // Use this for initialization
  void Start () {
    StartCoroutine (TextPrint(Textdia.GetComponent<Text>(), textMess, 0.1f, skipText));
    //isPrint = false;
  }

  // Update is called once per frame
  void Update () {

    //Debug.Log(textMess);
  }


  public IEnumerator TextPrint(Text output, string input, float delay, bool skip)
  {
    if (isPrint) yield break;
    isPrint = true;
     for (int i=0; i < input.Length; i++) 
    {
      if (skip) { output.text = input; break; }
      output.text = input.Substring(0, i);
      yield return new WaitForSeconds(delay);
    }
    //Debug.Log (output);
  }


}

unity error:
NullReferenceException: Object reference not set to an instance of an object
Dialogs+c__Iterator1.MoveNext () (at Assets/scripts/UI/Dialogs.cs:31)
UnityEngine.MonoBehaviour:StartCoroutine(IEnumerator)
Dialogs:Start() (at Assets/scripts/UI/Dialogs.cs:16)

Answer 1

for (int i=0; i < input.Length - 1; i++)
And you generally public IEnumerator TextPrintreturn nonsense. IEnumerator

Answer 2

Your textMess is equal to nothing .. you see.
not an empty string ... but straight ANYTHING.
and Nothing has a Length .
initialize the string at least ))
and in general, debugging works fine in the unit.
and nothing prevents Debug.Log Add and see what you send to the parameters