MahApps.Metro C# UI blocking

gistfile1.cs

public partial class MainWindow : MetroWindow
{
    public MainWindow()
    {
        InitializeComponent();          
        RegisterHandlers();
        this.Loaded += WindowLoaded;
    }

    void WindowLoaded(object sender, RoutedEventArgs e)
    {
        SynchronizeSyncthing();   
    }

    async private void SynchronizeSyncthing()
    {
        if (Process.GetProcessesByName("syncthing").Length > 0)
        {
            var result = await this.ShowMessageAsync("Oops", "Syncthing seems to be already running.\nWould you like to kill the existing instance to continue?", MessageDialogStyle.AffirmativeAndNegative, new MetroDialogSettings
            {
                AffirmativeButtonText = "Kill",
                NegativeButtonText = "Exit",
            });

            if (result == MessageDialogResult.Affirmative) {
                var progressCtrl = await this.ShowProgressAsync("Bare with me", "Killing existing syncthing processes");

                foreach (int attempt in Enumerable.Range(1, 5))
                {
                    if (attempt == 4)
                    {
                        progressCtrl.CloseAsync();
                        await this.ShowMessageAsync("Oops", "Can't kill existing Syncthing processes, please do it via Task Manager, and try again.", MessageDialogStyle.Affirmative);
                        Application.Current.Shutdown();
                    }

                    var processes = Process.GetProcessesByName("syncthing");

                    if (processes.Length == 0)
                    {
                        progressCtrl.CloseAsync();
                        break;
                    }

                    foreach (Process proc in processes)
                    {
                        proc.Kill();
                        proc.WaitForExit(2000);
                        proc.Refresh();
                    }
                }      
            } else {
                Application.Current.Shutdown();
            }
        }
        StartSyncthing();
    }

    private void StartSyncthing()
    {
            
    }
}