fix Xamarin iOS PlotViewRenderer crash #458

Source/OxyPlot.Xamarin.Forms.Platform.iOS/PlotViewRenderer.cs

@@ -40,7 +40,7 @@ namespace OxyPlot.Xamarin.Forms.Platform.iOS
                 Controller = this.Element.Controller
             };
 
-            if (this.Element.Model.Background.IsVisible())
+            if (this.Element.Model != null && this.Element.Model.Background.IsVisible())
             {
                 plotView.BackgroundColor = this.Element.Model.Background.ToUIColor();
             }