ShowDialog Can Only Be Used Once?

Hello all. I have created an application (WPF/C#) with several windows that I call with ShowDialog.

All works well unless I try to show two different windows, one after the other using ShowDialog on each. For some reason the second window will not show.

I created a new WPF application to reproduce the issue, leaving the default MainWindow. I added two additional windows. On each I added a single button that sets the DialogResult to true.

In App.cs I added the following....

using System.Windows;

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application
    {
        protected override void OnStartup(StartupEventArgs e)
        {
            base.OnStartup(e);


            Window1 w1 = new Window1();
            bool? result1 = w1.ShowDialog();
            if (result1.HasValue)
            {
                if (result1.Value)
                {
                    string x = "";
                }
            }
            Window2 w2 = new Window2();
            bool? result2 = w2.ShowDialog();
            if (result2.HasValue)
            {
                if (result2.Value)
                {
                    string y = "";
                }
            }

            
        }
    }
}

Could anyone clue me into why this line does not show the second window?

bool? result2 = w2.ShowDialog()

Regards,