I am getting the following exception:

"Nullable object must have a value"

Everything was working when I was using

StartupURI="MainWindow.xaml"

but I wanted to implement a login screen so I changed this to

Startup="Application_Startup" 

and then created the following method in App.xaml.cs:

    private void Application_Startup(object sender, StartupEventArgs e)
    {
        UpdateAccounts();
        bool result = true;
        ///*
        LoginWindow login = new LoginWindow();            
        result = login.ShowDialog().Value;
        /* */

        if (!result)
        {
            return;
        }

        MainWindow window = new MainWindow();
        bool main = window.ShowDialog().Value;
    }

Does anyone have any idea what is going on? Or any suggestions on what is the best practice for implementing login interface.