System.Runtime.InteropServices.COMException: 'Invalid window handle. (0x80070578) about winui-gallery HOT 1 OPEN

 picker.FileTypeChoices.Add("Rich Text", new List<string>() { ".rtf" });
 

 var hwnd = WindowNative.GetWindowHandle(this);  // App.m_window?
 InitializeWithWindow.Initialize(picker, hwnd);

 StorageFile file = await picker.PickSaveFileAsync();


 if (file != null)
 {
     // Prevent updates to the remote version of the file until we
     // finish making changes and call CompleteUpdatesAsync.
     CachedFileManager.DeferUpdates(file);
     // write to file
     using (Windows.Storage.Streams.IRandomAccessStream randAccStream =
         await file.OpenAsync(Windows.Storage.FileAccessMode.ReadWrite))
     {
         editor.Document.SaveToStream(Microsoft.UI.Text.TextGetOptions.FormatRtf, randAccStream);
     }

     // Let Windows know that we're finished changing the file so the
     // other app can update the remote version of the file.
     FileUpdateStatus status = await CachedFileManager.CompleteUpdatesAsync(file);
     if (status != FileUpdateStatus.Complete)
     {
         Windows.UI.Popups.MessageDialog errorBox =
             new Windows.UI.Popups.MessageDialog("File " + file.Name + " couldn't be saved.");
         await errorBox.ShowAsync();
     }
 }

// Dropdown of file types the user can save the file as
savePicker.FileTypeChoices.Add("Rich Text", new List<string>() { ".rtf" });

// Default file name if the user does not type one in or select a file to replace
savePicker.SuggestedFileName = "New Document";

StorageFile file = await savePicker.PickSaveFileAsync();
if (file != null)
{
    // Prevent updates to the remote version of the file until we
    // finish making changes and call CompleteUpdatesAsync.
    CachedFileManager.DeferUpdates(file);
    // write to file
    using (Windows.Storage.Streams.IRandomAccessStream randAccStream =
        await file.OpenAsync(Windows.Storage.FileAccessMode.ReadWrite))
    {
        editor.Document.SaveToStream(Microsoft.UI.Text.TextGetOptions.FormatRtf, randAccStream);
    }

    // Let Windows know that we're finished changing the file so the
    // other app can update the remote version of the file.
    FileUpdateStatus status = await CachedFileManager.CompleteUpdatesAsync(file);
    if (status != FileUpdateStatus.Complete)
    {
        Windows.UI.Popups.MessageDialog errorBox =
            new Windows.UI.Popups.MessageDialog("File " + file.Name + " couldn't be saved.");
        await errorBox.ShowAsync();
    }
}