react-native-webview/example/windows/WebViewWindows/App.xaml

<react:ReactApplication
    x:Class="WebViewWindows.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:WebViewWindows"
    xmlns:react="using:Microsoft.ReactNative">
    <Application.Resources>
        <XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls" />
    </Application.Resources>
</react:ReactApplication>