NotificationFlyout/NotificationFlyout.XamlHost/App.xaml

<xamlhost:XamlApplication
    x:Class="NotificationFlyout.XamlHost.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:xamlhost="using:Microsoft.Toolkit.Win32.UI.XamlHost">
</xamlhost:XamlApplication>