[iOS] Prevent NRE when page is switched on Appearing (#840)

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Unreported1.cs

@@ -0,0 +1,55 @@
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+#endif
+
+// Apply the default category of "Issues" to all of the tests in this assembly
+// We use this as a catch-all for tests which haven't been individually categorized
+#if UITEST
+[assembly: NUnit.Framework.Category("Issues")]
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.None, 0, "NRE when switching page on Appearing", PlatformAffected.iOS)]
+	public class Unreported1 : TestMasterDetailPage
+	{
+		static Unreported1 MDP;
+
+		class SplashPage : ContentPage
+		{
+			protected override void OnAppearing()
+			{
+				base.OnAppearing();
+
+				// You really shouldn't do this, but we shouldn't crash when you do it. :)
+				SwitchDetail();
+			}
+		}
+
+		protected override void Init()
+		{
+			MDP = this;
+
+			Master = new Page { Title = "Master" };
+			Detail = new SplashPage();
+		}
+
+		public static void SwitchDetail()
+		{
+			MDP.Detail = new ContentPage { Content = new Label { Text = "If this did not crash, this test has passed." }, Padding = 20 };
+		}
+
+#if UITEST
+		[Test]
+		public void Unreported1Test()
+		{
+			RunningApp.Screenshot("ensure there is no crash");
+		}
+#endif
+	}
+}

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

@@ -269,6 +269,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla52533.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla53362.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla45874.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Unreported1.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla53909.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)_Template.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue1028.cs" />

Xamarin.Forms.Platform.iOS/Renderers/PageRenderer.cs

@@ -200,6 +200,9 @@ namespace Xamarin.Forms.Platform.iOS
 
 		void UpdateStatusBarPrefersHidden()
 		{
+			if (Element == null)
+				return;
+
 			var animation = ((Page)Element).OnThisPlatform().PreferredStatusBarUpdateAnimation();
 			if (animation == PageUIStatusBarAnimation.Fade || animation == PageUIStatusBarAnimation.Slide)
 				UIView.Animate(0.25, () => SetNeedsStatusBarAppearanceUpdate());