how to solve null reference on file browser script

i'm new to Unity and C# and trying to make an game in which users could upload images and i'm using an free asset named Simple File Browser by Graces Games. it is an asset to browser files in the game, i'm trying to make use of it's Filebrowser class but not able to get any help.

here is my code

 using System.Collections;
 using UnityEngine.UI;
 using System.Collections.Generic;
 using UnityEngine;
 using System.Runtime.Serialization.Formatters.Binary;
 using System.IO;
 using System;
 
 namespace GracesGames.SimpleFileBrowser.Scripts
 {
     public class explorer_script : MonoBehaviour
     {
         private GameObject FileBrowserPrefab;
         public string[] FileExtensions = { "jpeg", "jpg", "png" };
         private GameObject _loadedImage;
         public RawImage image;
         public bool PortraitMode;
         void start()
         {
             
             PortraitMode = true;
         }
 
         public void OpenFileBrowser()
         {
             OpenFileBrowser(FileBrowserMode.Load);
         }
 
         private void OpenFileBrowser(FileBrowserMode fileBrowserMode)
         {
             FileBrowserPrefab = GameObject.Find("FileBrowser");
             // Create the file browser and name it
             GameObject fileBrowserObject = Instantiate(FileBrowserPrefab, transform);
             fileBrowserObject.name = "FileBrowser";
             // Set the mode to save or load
             FileBrowser fileBrowserScript = fileBrowserObject.GetComponent<FileBrowser>();
             fileBrowserScript.SetupFileBrowser(PortraitMode ? ViewMode.Portrait : ViewMode.Landscape);
  
             fileBrowserScript.OpenFilePanel(FileExtensions);
             fileBrowserScript.OnFileSelect += LoadFileUsingPath;
  
         }
 
         // Loads a file using a path
         private void LoadFileUsingPath(string path)
         {
             if (path.Length != 0)
             {
                 BinaryFormatter bFormatter = new BinaryFormatter();
                 // Open the file using the path
                 FileStream file = File.OpenRead(path);
                 // Convert the file from a byte array into a RawImage
                 RawImage fileData = bFormatter.Deserialize(file) as RawImage;
                 // We're done working with the file so we can close it
                 file.Close();
                 
             }
             else
             {
                 Debug.Log("Invalid path given");
             }
         }
 
     }
 }

so what i'm confused is that the FileBrowserPrefab requires an gameobject, how i'm supposed to attach prefab to it?

So what i do is that i create a gameObject add my script to it (which stated above), and then on the button onClick() i attach the gameObject (on which i have attached the script) and call the OpenFileBrowser() method?

but it's giving me following error..

NullReferenceException: Object reference not set to an instance of an object GracesGames.SimpleFileBrowser.Scripts.explorer_script.OpenFileBrowser (FileBrowserMode fileBrowserMode) (at Assets/explorer_script.cs:37) GracesGames.SimpleFileBrowser.Scripts.explorer_script.OpenFileBrowser () (at Assets/explorer_script.cs:26) UnityEngine.Events.InvokableCall.Invoke () (at C:/buildslave/unity/build/Runtime/Export/UnityEvent.cs:166) UnityEngine.Events.UnityEvent.Invoke () (at C:/buildslave/unity/build/Runtime/Export/UnityEvent_0.cs:58) UnityEngine.UI.Button.Press () (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:36) UnityEngine.UI.Button.OnPointerClick (UnityEngine.EventSystems.PointerEventData eventData) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:45) UnityEngine.EventSystems.ExecuteEvents.Execute (IPointerClickHandler handler, UnityEngine.EventSystems.BaseEventData eventData) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:50) UnityEngine.EventSystems.ExecuteEvents.Execute[IPointerClickHandler] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.EventFunction`1 functor) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:261) UnityEngine.EventSystems.EventSystem:Update()

please any help will be really appreciated?

thanks..