Modal dialog native in Tabbed Page

[bitworked]

I have a tabbed page wrapped in a navigationbar and I want to open a native modal window. With version 8 I can't do it. This is my code:

` <TabbedPage.Children>

<x:Arguments>
<uc:MainPageView />
</x:Arguments>
<NavigationPage.IconImageSource>

</NavigationPage.IconImageSource>

    <NavigationPage Title="{x:Static resource:AppResources.Fav}">
        <x:Arguments>
            <uc:FavoritesView />
        </x:Arguments>
        <NavigationPage.IconImageSource>
            <FontImageSource FontFamily="FaLight-300" Glyph="&#xf0c0;"></FontImageSource>
        </NavigationPage.IconImageSource>

    </NavigationPage>

</TabbedPage.Children>`

Open the tabbed page like
await _navigationservice.NavigateAsync("/MainTabView?selectedTab=MainPageView");

and I open the window as usual but not work
await _navigationservice.NavigateAsync("NavigationBar/AppInfoView", null, useModalNavigation: true, true);

Can you help me?
Thanks