我的代碼如下所示：引導程序public class Bootstrapper : BootstrapperBase{    private SimpleContainer _container = new SimpleContainer();    public Bootstrapper()    {        Initialize();    }    protected override void OnStartup(object sender, StartupEventArgs e)    {        base.OnStartup(sender, e);        DisplayRootViewFor<ShellViewModel>();    }    protected override void Configure()    {        _container.Singleton<IEventAggregator, EventAggregator>();        _container.Singleton<IWindowManager, WindowManager>();        _container.RegisterPerRequest(typeof(ShellViewModel), null, typeof(ShellViewModel));       }    protected override object GetInstance(Type service, string key)    {        return _container.GetInstance(service, key);    }    protected override IEnumerable<object> GetAllInstances(Type serviceType)    {        return _container.GetAllInstances(serviceType);    }    protected override void BuildUp(object instance)    {        _container.BuildUp(instance);    }}我的ShellViewModel樣子是這樣的：ShellViewModel.cspublic class ShellViewModel : Conductor<Screen>{    public ShellViewModel    {        var aViewModel = IoC.Get<AViewModel>();        ActivateItem(aViewModel);    }}但是每當我運行程序時，都會顯示一個空白屏幕。當我調試它時，它說aViewModel是null.有什么問題Bootstrapper嗎？