Problem with a Unity tutorial

Hi, i´m trying to do something like this video: https://www.youtube.com/watch?v=_ijhS6aezQc But i´m not using all these variables and always i receive the same error:

NullReferenceException: Object reference not set to an instance of an object CreateScrollList.PopulateList () (at Assets/Scripts/MainMenu/CreateScrollList.cs:29) CreateScrollList.Start () (at Assets/Scripts/MainMenu/CreateScrollList.cs:21)

The problem is when i tried to get the sample button, i don´t know why, could some one help to me? I put in strong the problem.

Here is my CreateScrollList. Thank you so much!

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 using System.Collections.Generic;
 using System.IO;
 
 [System.Serializable]
 public class Item {
     public string name;
     public Button.ButtonClickedEvent thingToDo;
 }
 
 public class CreateScrollList : MonoBehaviour {
     
     public GameObject sampleButton;
     public List<Item> itemList;
     
     public Transform contentPanel;
     
     void Start () {
         PopulateList ();
     }
     
     void PopulateList () {
         itemList = getFiles ();
         foreach (var item in itemList) {
             GameObject newButton = Instantiate (sampleButton) as GameObject;
             **SampleButton button = newButton.GetComponent <SampleButton> ();
             button.nameLabel.text = item.name;**
             newButton.transform.SetParent (contentPanel);
         }
     }
 
 public List<Item> getFiles(){
 ...
 }