fix: XamlRoot may be null when the SKXamlCanvas is unloaded (#2854)

* fix: XamlRoot may be null when the SKXamlCanvas is unloaded

* chore: Adjust more xamlroot uses

---------

Co-authored-by: Matthew Leibowitz <mattleibow@live.com>

source/SkiaSharp.Views/SkiaSharp.Views.WinUI/SKXamlCanvas.cs

@@ -101,7 +101,7 @@ namespace SkiaSharp.Views.UWP
 		private void OnXamlRootChanged(XamlRoot xamlRoot = null, XamlRootChangedEventArgs e = null)
 		{
 			var root = xamlRoot ?? XamlRoot;
-			Dpi = root.RasterizationScale;
+			Dpi = root?.RasterizationScale ?? 1.0;
 			Invalidate();
 		}
 #else
@@ -141,7 +141,10 @@ namespace SkiaSharp.Views.UWP
 				return;
 
 #if WINDOWS
-			XamlRoot.Changed -= OnXamlRootChanged;
+			if(XamlRoot != null)
+			{
+				XamlRoot.Changed -= OnXamlRootChanged;
+			}
 #else
 			var display = DisplayInformation.GetForCurrentView();
 			display.DpiChanged -= OnDpiChanged;