How to fix when I click the Open file button in the file picker it crashes?

Question

I am trying to create a file picker using WInUI3. My app crashes when I click on the open file button. Below is the photo of the error....

It says :

  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=Microsoft.Windows.SDK.NET
  StackTrace:
   at WinRT.Interop.IWindowNativeMethods.get_WindowHandle(Object _obj)
   at WinRT.Interop.WindowNative.GetWindowHandle(Object target)
   at WINUITMP.Views.MainPage.<PickAFileButton_Click>d__4.MoveNext() in C:\Users\User\source\repos\WINUITMP\Views\MainPage.xaml.cs:line 95

I just added the WindowsHelper class from This Link which is in github Please help to fix this below is my C# code

using Microsoft.UI.Xaml.Controls;
using CommunityToolkit.WinUI.Controls;
using WINUITMP.ViewModels;
using WINUITMP.Views;
using Windows.Storage.Pickers;
using WinRT.Interop;
using CommunityToolkit.WinUI.Helpers;
using Windows.Storage;
using Microsoft.UI;
using Microsoft.UI.Windowing;
using System;
using System.Collections.Generic;
using AppUIBasics.Helper;
using Microsoft.UI.Xaml.Media;

namespace AppUIBasics.Helper
{
    // Helper class to allow the app to find the Window that contains an
    // arbitrary UIElement (GetWindowForElement).  To do this, we keep track
    // of all active Windows.  The app code must call WindowHelper.CreateWindow
    // rather than "new Window" so we can keep track of all the relevant
    // windows.  In the future, we would like to support this in platform APIs.
    public class WindowHelper
    {
        public static Window CreateWindow()
        {
            Window newWindow = new Window
            {
                SystemBackdrop = new MicaBackdrop()
            };
            TrackWindow(newWindow);
            return newWindow;
        }

        public static void TrackWindow(Window window)
        {
            window.Closed += (sender, args) => {
                ActiveWindows.Remove(window);
            };
            ActiveWindows.Add(window);
        }

        public static AppWindow GetAppWindow(Window window)
        {
            var hWnd = WindowNative.GetWindowHandle(window);
            WindowId wndId = Win32Interop.GetWindowIdFromWindow(hWnd);
            return AppWindow.GetFromWindowId(wndId);
        }

        public static Window? GetWindowForElement(UIElement element)
        {
            if (element.XamlRoot != null)
            {
                foreach (Window window in ActiveWindows)
                {
                    if (element.XamlRoot == window.Content.XamlRoot)
                    {
                        return window;
                    }
                }
            }
            return null;
        }

        public static List<Window> ActiveWindows { get; } = new List<Window>();
    }
}

namespace WINUITMP.Views
{
    public sealed partial class MainPage : Page
    {

        public MainViewModel ViewModel
        {
            get;
        }

        public MainPage()
        {
            ViewModel = App.GetService<MainViewModel>();
            InitializeComponent();
        }
        private async void PickAFileButton_Click(object sender, RoutedEventArgs e)
        {
            // Clear previous returned file name, if it exists, between iterations of this scenario
            PickAFileOutputTextBlock.Text = "";

            // Create a file picker
            var openPicker = new Windows.Storage.Pickers.FileOpenPicker();

            // Retrieve the window handle (HWND) of the current WinUI 3 window.
            var window = WindowHelper.GetWindowForElement(this);
            var hWnd = WinRT.Interop.WindowNative.GetWindowHandle(window);

            // Initialize the file picker with the window handle (HWND).
            WinRT.Interop.InitializeWithWindow.Initialize(openPicker, hWnd);

            // Set options for your file picker
            openPicker.ViewMode = PickerViewMode.Thumbnail;
            openPicker.FileTypeFilter.Add("*");

            // Open the picker for the user to pick a file
            var file = await openPicker.PickSingleFileAsync();
            if (file != null)
            {
                PickAFileOutputTextBlock.Text = "Picked file: " + file.Name;
            }
            else
            {
                PickAFileOutputTextBlock.Text = "Operation cancelled.";
            }
        }
    }
}

I need a way to fix this , I dont know why it crashes.