WPF how to focus the previous Window?

Good people,

I have a window in ToolBox WindowStyle="ToolWindow" which serves only to inform the user what is happening in BackgroundWorker

But I'm having a problem, for example I have this code that is in the Activated event:

private void WindowDocumentPriceCheck_Activated(object sender, EventArgs e)
{
    this.Activated -= WindowDocumentPriceCheck_Activated;
    BackgroundWorker workerFillTheLists = new BackgroundWorker();
    workerFillTheLists.DoWork += this.workerFillTheLists_DoWork;
    workerFillTheLists.RunWorkerCompleted += this.workerFillTheLists_RunWorkerCompleted;
    this.windowWaitForWorker = new WindowWaitForWorker("Informação", "Por favor aguarde!");
    this.windowWaitForWorker.WindowStartupLocation = WindowStartupLocation.CenterScreen;
    this.windowWaitForWorker.Closed += this.WindowWaitForWorker_Closed;
    workerFillTheLists.RunWorkerAsync();
    // Quero que o utilizador não tenha a hipotesses de carregar nos botões da janela anterior dai estar com ShowDialog();
    this.windowWaitForWorker.ShowDialog();
}

The problem is when the Window windowWaitForWorker quits it does not focus the previous window.

I have tried several things, this add the event Closed activate or give focus and nothing

private void WindowWaitForWorker_Closed(object sender, EventArgs e)
{
    this.Activate();
    this.Topmost = true;  // important
    this.Topmost = false; // important
    this.Focus();
    this.textBoxBarCode.Focus();
}

Here is the code that is behind WindowWaitForWorker

public partial class WindowWaitForWorker : Window
{
    private const int GWL_STYLE = -16;
    private const int WS_SYSMENU = 0x00080000;

    [DllImport("user32.dll", SetLastError = true)]
    private static extern int GetWindowLong(IntPtr hWnd, int nIndex);

    [DllImport("user32.dll")]
    private static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);

    /// <summary>
    /// Consctuctor that will receive the title and message to show
    /// </summary>
    /// <param name="title">Title for the window</param>
    /// <param name="message">Message for the user</param>
    public WindowWaitForWorker(string title, string message)
    {
        InitializeComponent();
        this.Title = title;
        this.textBoxMessage.Text = message;
        this.Loaded += this.WindowWaitForWorker_Loaded;
    }

    /// <summary>
    /// Updates the message for the user to see
    /// </summary>
    /// <param name="message">Message for the user</param>
    public void UpdateMessage(string message)
    {
        this.textBoxMessage.Text = message;
    }

    private void WindowWaitForWorker_Loaded(object sender, RoutedEventArgs e)
    {
        var hwnd = new WindowInteropHelper(this).Handle;
        SetWindowLong(hwnd, GWL_STYLE, GetWindowLong(hwnd, GWL_STYLE) & ~WS_SYSMENU);
    }
}

Nothing you've tried worked out because you wanted the window to focus as soon as WindowWaitForWorker is closed AKA when BackgroundWorker