Estou usando o código a seguir para abrir uma janela em um thread separado

public partial class App : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);
    }

    private void button1_Click(object sender, RoutedEventArgs e)
    {
        Thread newWindowThread = new Thread(new ThreadStart(() =>
        {
            // Create and show the Window
            Config tempWindow = new Config();
            tempWindow.Show();
            // Start the Dispatcher Processing
            System.Windows.Threading.Dispatcher.Run();
        }));

        // Set the apartment state
        newWindowThread.SetApartmentState(ApartmentState.STA);
        // Make the thread a background thread
        newWindowThread.IsBackground = true;
        // Start the thread
    }
}

Se eu usar esse código em um método, ele funcionará. Mas quando eu o uso da seguinte maneira, recebo um erro:

public partial class App : Application
{
    #region Instance Variables
    private Thread newWindowThread;

    #endregion

    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);
        newWindowThread = new Thread(new ThreadStart(() =>
        {
            // Create and show the Window
            Config tempWindow = new Config();
            tempWindow.Show();
            // Start the Dispatcher Processing
            System.Windows.Threading.Dispatcher.Run();
        }));
    }

    private void button1_Click(object sender, RoutedEventArgs e)
    {
        // Set the apartment state
        newWindowThread.SetApartmentState(ApartmentState.STA);
        // Make the thread a background thread
        newWindowThread.IsBackground = true;
        // Start the thread
    }
}

Emite o seguinte erro:

System.Threading.ThreadStateException
The state of the thread was not valid to execute the operation

Qual é a causa disso?