add error notification when a RecipeView is clicked without the recipe being found

Views/HomePage.xaml.cs

@@ -20,8 +20,13 @@ public partial class HomePage : ContentPage
         {
             layout.Children.Add(new RecipeView(info, () =>
             {
-                Recipe recipe = service.GetRecipe(info);
+                Recipe? recipe = service.GetRecipe(info);
+                if (recipe != null)
                     Shell.Current.Navigation.PushAsync(new RecipePage(recipe, notifier, preferences, 1));
+                else
+                {
+                    notifier.Error("Could not find recipe");
+                }
             }));
         }