fix(reg): Fix null anotation

src/Uno.UI.Runtime.Skia.Linux.FrameBuffer/FramebufferHost.cs

@@ -95,7 +95,7 @@ namespace Uno.UI.Runtime.Skia
 		{
 			_isDispatcherThread = true;
 
-			ApiExtensibility.Register(typeof(Uno.ApplicationModel.Core.ICoreApplicationExtension), o => _coreApplicationExtension);
+			ApiExtensibility.Register(typeof(Uno.ApplicationModel.Core.ICoreApplicationExtension), o => _coreApplicationExtension!);
 			ApiExtensibility.Register(typeof(Windows.UI.Core.ICoreWindowExtension), o => new CoreWindowExtension(o));
 			ApiExtensibility.Register(typeof(Windows.UI.ViewManagement.IApplicationViewExtension), o => new ApplicationViewExtension(o));
 			ApiExtensibility.Register<Application>(typeof(Uno.UI.Xaml.IApplicationExtension), o => new ApplicationExtension(o));