NullReferenceException: Object reference not set to an instance of an object

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
 
public class GameHelper : MonoBehaviour
{
    public InputField Input;
    public Text TextBlock;
 
    private PlayerHelper _currentPlayer;
    public PlayerHelper CurrentPlayer
    {
        get { return _currentPlayer; }
        set { _currentPlayer = value; }
    }
    public void Send()
    {
        CurrentPlayer.CmdSend(Input.text);
    }
}

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Networking;
 
public class PlayerHelper : NetworkBehaviour
{
    GameHelper _gameHelper;
    // Start is called before the first frame update
    void Start()
    {
        _gameHelper = GameObject.FindObjectOfType<GameHelper>();
        if (isLocalPlayer)
        {
            _gameHelper.CurrentPlayer = this;
        }
    }
 
    [Command]
    public void CmdSend(string message)
    {
        Debug.Log("Send =" + message);
        RpcSend(message);
    }
    [ClientRpc]
    public void RpcSend(string message)
    {
        _gameHelper.TextBlock.text += System.Environment.NewLine + message;
    }