object reference not set to an instance of an object uipage.setselecteduitodefault ()

Question

I'm new in unity, and I'm not good at scripting;

I just recognize there is a problem with

SetSelectedUIToDefault()

The error is nullreferenceexception

Thanks for helping me;

Here is script:

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

/// <summary>
/// This class stores relevant information about a page of UI
/// </summary>
public class UIPage : MonoBehaviour
{
    [Tooltip("The default UI to have selected when opening this page")]
    public GameObject defaultSelected;


/// <summary>
/// Description:
/// Sets the selected UI selectable to the default defined by this UIPage
/// Inputs:
/// none
/// Returns:
/// void (no return)
/// </summary>
public void SetSelectedUIToDefault()
{
    if (defaultSelected != null)
    {
        GameManager.instance.uiManager.eventSystem.SetSelectedGameObject(defaultSelected);
        GameManager.instance.uiManager.eventSystem.SetSelectedGameObject(null);
    }
    
}
}