android - Xamarin.Forms - Screen black when using NavigationPage -

i adding navigationpage in xamarin.forms app. after adding navigationpage, screen black , not showing target page.

this new project, code quite simple.

public partial class app : application {     public app()     {         initializecomponent();         mainpage = new navigationpage(new hostingpage());     } 

the hostingpage contentpage label, entry , button. expected see hostingpage, end seeing black screen.

in mainactivity of android project, extending default:

[activity(label = "meeting.droid", icon = "@drawable/icon", theme = "@style/mytheme", mainlauncher = true, configurationchanges = configchanges.screensize | configchanges.orientation)] public class mainactivity : global::xamarin.forms.platform.android.formsappcompatactivity {     protected override void oncreate(bundle bundle)     {         tablayoutresource = resource.layout.tabbar;         toolbarresource = resource.layout.toolbar;          base.oncreate(bundle);          global::xamarin.forms.forms.init(this, bundle);          loadapplication(new app()); 

try removing theme = "@style/mytheme" mainactivity of android project