I can't set player names using input field (following tutorials in official web site)

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using Photon.Pun;
using Photon.Realtime;
using UnityEngine.UI;

[RequireComponent(typeof(InputField))]
public class NicknameInputField : MonoBehaviour
 {
        #region Private Constants


	 // Store the PlayerPref Key to avoid typos
	 const string playerNamePrefKey = "PlayerName";


        #endregion


        #region MonoBehaviour CallBacks


	 /// <summary>
	 /// MonoBehaviour method called on GameObject by Unity during initialization phase.
	 /// </summary>
	 void Start () {


		 string defaultName = string.Empty;
		 InputField _inputField = this.GetComponent<InputField>();
		 if (_inputField!=null)
		 {
			 if (PlayerPrefs.HasKey(playerNamePrefKey))
			 {
				 defaultName = PlayerPrefs.GetString(playerNamePrefKey);
				 _inputField.text = defaultName;
			 }
		 }


		 PhotonNetwork.NickName =  defaultName;
	 }


        #endregion


        #region Public Methods


	 /// <summary>
	 /// Sets the name of the player, and save it in the PlayerPrefs for future sessions.
	 /// </summary>
	 /// <param name="value">The name of the Player</param>
	 public void SetPlayerName(string value)
	 {
		 // #Important
		 if (string.IsNullOrEmpty(value))
		 {
			 Debug.LogError("Player Name is null or empty");
			 return;
		 }
		 PhotonNetwork.NickName = value;


		 PlayerPrefs.SetString(playerNamePrefKey,value);
	 }


        #endregion
 }