Can't show a wpf win called by static async function

Question

I've an C#/Wpf application who needs to call a static class as soon as possible ( in Initialized event ).

The static class starts a process and during this process I need to have some WPF windows.

If all process in my static class is synchronous => no problem all works fine!

But I need to have the process asynchronous to not block the application.

As soon as I use a async function from library or use async/await, my windows are created but not show, I just see an icon in task bar and a black rectangle if I put the mouse over this icon.

If I do the same (asynchronous) from a button in my application all works fine !

I try many, many thinks with thread or dispatcher but nothing works.

Here is some code snippet:

Application:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private void OnMainInitialized( object sender, EventArgs e ) // THIS DON'T WORKS!
    {
        Updater.Start( http://10.63.4.48:8080/UpdaterFiles/QM_Updater_Test/QM_Updater_Test.xml" );
    }

    private void Button_Click( object sender, RoutedEventArgs e ) // THIS WORKS!
    { 
        Updater.Start( "http://10.63.4.48:8080/UpdaterFiles/QM_Updater_Test/QM_Updater_Test.xml" );
    }

}

And static class :

public static class Updater 
{
public static void Start( string Url )
        
            //Ask_Win Dlg = new Ask_Win(); //IF UNCOMMENTED ALWAYS SHOW THE WINDOW (sync or async !!)
            //Dlg.Show();

            string LocalXml = System.IO.Path.GetDirectoryName( System.Reflection.Assembly.GetExecutingAssembly().Location ) + "\\Updates.xml";

            if ( File.Exists( LocalXml ) ) ParseLocalXml( LocalXml );

            if ( !CheckForInternetConnection() ) return;

            m_Xml_Uri = new Uri( Url );

            if ( sync ) { // SYNCHRONNOUS WAY => ALWAYS OK

                if ( CheckUpdateSync() ) {

                    bool DoTheJob = true;

                    if ( !m_MandatoryUpdate ) DoTheJob = AskForUpdate();

                    if ( DoTheJob ) DoUpdate();
                }
            }
            else 
                GetRemoteXmlAsync();  // SYNCHRONOUS WAY => ALWAYS BAD IF CALLED BY APPLICATION INITITIALIZED

        }

private static void GetRemoteXmlAsync()
        {
            using ( WebClient Wc = GetWebClient( null ) ) {

                Wc.DownloadStringCompleted += OnRemoteXmlCompleted;

                Wc.DownloadStringAsync( m_Xml_Uri );
            }

        }

private static void OnRemoteXmlCompleted( object sender, DownloadStringCompletedEventArgs EventArgs )
        {
            try {

                string Xml = EventArgs.Result;

                if ( CheckRemoteXml( Xml ) ) {

                    bool DoTheJob = true;

                    if ( !m_MandatoryUpdate ) DoTheJob = AskForUpdate();

                    if ( DoTheJob ) DoUpdate();
                }

            }catch (Exception e ) {

                ShowException( e );

            }
        }

//HERE IS THE FATAL FUNCTION !!!
        private static bool AskForUpdate()
        {
            bool Ret = false;

            using ( Ask_Win AskWin = new Ask_Win() ) {
            
                if ( !string.IsNullOrEmpty( m_UpdateText ) ) AskWin.SetText( m_UpdateText );

                if ( AskWin.ShowDialog() == true ) Ret = true;
            }

            return Ret;
        }
}

To resume :

• If AskForUpdate is called without any async function => all ik OK
• If AskForUpdate is called after one async function => ShowDialog show nothing but the modal win is created and started ( => no action possible because it's a modal dialog)
• If AskForUpdate is called after one async function BUT from a button is main application => all is OK

I try these kind of thinks : Dispatcher.Invoke( () => ShowDialog() ); from static class, from wpf form, ... I try to store the dispatcher in the beginning of the function , ....

No results until now !!!

Any ideas are welcome !

Thanks