Add null check in iOS ShadowEffect OnDetached (#5952)

Added a null check in iOS ShadowEffect OnDetached
fixes #5951

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue5951.cs

@@ -0,0 +1,92 @@
+using System;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+using Xamarin.Forms.PlatformConfiguration;
+using Xamarin.Forms.PlatformConfiguration.iOSSpecific;
+
+#if UITEST
+using Xamarin.Forms.Core.UITests;
+using Xamarin.UITest;
+using NUnit.Framework;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 5951, "App Crashes On Shadow Effect's OnDetached On Button That's Never Visible", PlatformAffected.iOS)]
+	public class Issue5951 : TestContentPage
+	{
+		protected override void Init()
+		{
+			var instructionsLabel = new Label { Text = "Press the push page button. If everything works, you'll see the word success below.", FontSize = 16 };
+
+			var resultsLabel = new Label { Text = string.Empty, FontSize = 16 };
+
+			var pushButton = new Button
+			{
+				Text = "Push page",
+				Command = new Command(async () =>
+				{
+					var shadowPage = new PageWithShadowButton();
+
+					await Navigation.PushAsync(shadowPage);
+
+					try
+					{
+						await shadowPage.Navigation.PopAsync();
+
+						resultsLabel.Text = "Success";
+					}
+					catch (NullReferenceException)
+					{
+						resultsLabel.Text = "Error";
+					}
+				})
+			};
+
+			Content = new StackLayout
+			{
+				Children = {
+					instructionsLabel,
+		  			resultsLabel,
+					pushButton
+				}
+			};
+
+
+		}
+
+#if UITEST
+		[Test]
+		public void Issue5951Test()
+		{
+			RunningApp.Tap(q => q.Marked("Push page"));
+			RunningApp.WaitForElement(q => q.Marked("Push page"));
+
+			RunningApp.WaitForElement(q => q.Marked("Success"));
+		}
+#endif
+	}
+
+	[Preserve(AllMembers = true)]
+	public class PageWithShadowButton : ContentPage
+	{
+		public PageWithShadowButton()
+		{
+			var shadowButton = new Button { Text = "Never Visible", IsVisible = false };
+
+			shadowButton.On<iOS>()
+				.SetIsShadowEnabled(true)
+				.SetShadowColor(Color.Black)
+				.SetShadowOffset(new Size(10, 10))
+				.SetShadowOpacity(0.2);
+
+			Content = new StackLayout
+			{
+				Children = { shadowButton }
+			};
+		}
+	}
+
+
+}

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems

@@ -915,6 +915,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Issue4356.cs">
       <DependentUpon>Issue4356.xaml</DependentUpon>
     </Compile>
+    <Compile Include="$(MSBuildThisFileDirectory)Issue5951.cs" />
   </ItemGroup>
   <ItemGroup>
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)Bugzilla22229.xaml">
@@ -1162,7 +1163,7 @@
   <ItemGroup>
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)A11yTabIndex.xaml">
       <SubType>Designer</SubType>
-      <Generator>MSBuild:Compile</Generator>
+      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
   </ItemGroup>
 </Project>

Xamarin.Forms.Platform.iOS/ShadowEffect.cs

@@ -15,9 +15,13 @@ namespace Xamarin.Forms.Platform.iOS
 		protected override void OnDetached()
 		{
 			var layer = ShadowView.Layer;
+
+			if (layer != null)
+			{
 				layer.ShadowColor = Color.Transparent.ToCGColor();
 				layer.ShadowOpacity = 0;
 			}
+		}
 
 		protected override void OnElementPropertyChanged(PropertyChangedEventArgs args)
 		{