Loading custom page first in Unity3d

Question

im also working in something like this : 
mainPage.xaml content the unity scripts 
BasicPage.xaml content button
it's not a probleme to show unity's splash screen .. my probleme is to load the BasicPage.xaml before unity loads , and when the button is clicked , the mainPages.xaml loads
i'h tried to load the basicpage with this code in app.xaml.cs :
protected override void OnLaunched(LaunchActivatedEventArgs args)
            var mainPage = new BasicPage();
Window.Current.Content = mainPage;
Window.Current.Activate();
and is the button void : 
private void Button_Click(object sender, RoutedEventArgs a)
            this.Frame.Navigate(typeof(MainPage));
it's return a NULLEXCEPTION At this line  this.Frame.Navigate(typeof(MainPage));

Answer 1

hi :)
im working on a project in unity3d , and i build a solution to windows app store , that i opened it using visual studio community , then when i run it , automaticly the first page is the page of the game ..
i want to add a new page with a button , and when i click it navigate to the page that content the game ..
im stuck , i need help