How do I open a WPF window from Visual Studio add-in ?

Question

If I do this code in a clean addins template in Visual Studio 2008:

public void Exec(....
{
....
  Application app = new Application();
  MainWindows _mainWindow = new MainWindows();
  _mainWindow.Initialize(@"C:\");       
  app.Run(_mainWindow);
....
}

Opens the window right first time, BUT I can't open the window the second time I try from Tools menu. Why?