WPF application without a window loading automatically

I have set up a WPF application so that it does not load a window automatically by modifying the app.xaml.cs as shown below and by setting the build action to Page. I have also removed the starturl from the app.xaml. I have introduced a controller class from where the application starts as you can see in the code below and in the constructor for the controller I have created and opened a new window (I deleted the original MainWindow which was produced automatically when opening the application. However I then decided I needed a window and added another one. Anyway the problem is that after opening the new window that I have added in the controller I then close the window and the Application terminates? Why? I don't want the application to terminate and I don't understand why it is terminating on closing the window. Any help greatly appreciated.

using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Threading.Tasks;
using System.Windows;

namespace UserTraining
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application
    {
        App()
        {
            InitializeComponent();
        }

        [STAThread]
        static void Main()
        {            
            App app = new App();
            UserTrainingController control = new UserTrainingController();
            app.Run();
        }
    
    }
}
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;

namespace UserTraining
{
    class UserTrainingController
    {
        public UserTrainingController()
        {
            IntroductionWindow w = new IntroductionWindow();
            w.Show();
        }
    }
}