Fix issue #289: unable to use StockIcons in Gtk ListView

Xwt.Gtk/Xwt.GtkBackend/ImageHandler.cs

@@ -404,12 +404,14 @@ namespace Xwt.GtkBackend
 			Gdk.CairoHelper.SetSourcePixbuf (ctx, img, 0, 0);
 
 			#pragma warning disable 618
-			using (var pattern = (Cairo.SurfacePattern)ctx.Source) {
-				if (idesc.Size.Width > img.Width || idesc.Size.Height > img.Height) {
-					// Fixes blur issue when rendering on an image surface
-					pattern.Filter = Cairo.Filter.Fast;
-				} else
-					pattern.Filter = Cairo.Filter.Good;
+			using (var pattern = ctx.Source as Cairo.SurfacePattern) {
+				if (pattern != null) {
+					if (idesc.Size.Width > img.Width || idesc.Size.Height > img.Height) {
+						// Fixes blur issue when rendering on an image surface
+						pattern.Filter = Cairo.Filter.Fast;
+					} else
+						pattern.Filter = Cairo.Filter.Good;
+				}
 			}
 			#pragma warning restore 618