[iOS] Handle user popping pages out of order (#934)

* [iOS] Handle when user can call pop a page that was already disposed/removed

* [Controls] Fix bug number

Xamarin.Forms.ControlGallery.iOS/CustomRendererBugzila38731.cs

@@ -0,0 +1,29 @@
+using UIKit;
+using Xamarin.Forms;
+using Xamarin.Forms.ControlGallery.iOS;
+using Xamarin.Forms.Controls.Issues;
+
+[assembly: ExportRenderer(typeof(Bugzilla38731), typeof(CustomRendererBugzila38731))]
+[assembly: ExportRenderer(typeof(Bugzilla38731.PageTwo), typeof(CustomRendererBugzila38731))]
+[assembly: ExportRenderer(typeof(Bugzilla38731.PageThree), typeof(CustomRendererBugzila38731))]
+[assembly: ExportRenderer(typeof(Bugzilla38731.PageFour), typeof(CustomRendererBugzila38731))]
+
+namespace Xamarin.Forms.ControlGallery.iOS
+{
+	public class CustomRendererBugzila38731 : Platform.iOS.PageRenderer
+	{
+		public override void ViewWillAppear(bool animated)
+		{
+			base.ViewWillAppear(animated);
+
+			if (NavigationController.ViewControllers.Length > 1)
+			{
+				NavigationController.TopViewController.NavigationItem.SetLeftBarButtonItem(new UIBarButtonItem(
+					UIImage.FromFile("bank.png"), UIBarButtonItemStyle.Plain, (sender, args) =>
+					{
+						NavigationController.PopViewController(true);
+					}), true);
+			}
+		}
+	}
+}

Xamarin.Forms.ControlGallery.iOS/Xamarin.Forms.ControlGallery.iOS.csproj

@@ -169,6 +169,7 @@
     <Compile Include="Properties\AssemblyInfo.cs" />
     <None Include="packages.config" />
     <Compile Include="CustomRenderers.cs" />
+    <Compile Include="CustomRendererBugzila38731.cs" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\Xamarin.Forms.Controls\Xamarin.Forms.Controls.csproj">

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla38731.cs

@@ -0,0 +1,129 @@
+using System;
+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.Bugzilla, 38731, "Xamarin.Forms.Platform.iOS.NavigationRenderer.GetAppearedOrDisappearedTask NullReferenceExceptionObject", PlatformAffected.Default)]
+	public class Bugzilla38731 : TestContentPage // or TestMasterDetailPage, etc ...
+	{
+		protected override void Init()
+		{
+			var label = new Label();
+			label.Text = "Page one...";
+			label.HorizontalTextAlignment = TextAlignment.Center;
+
+			var button = new Button();
+			button.AutomationId = "btn1";
+			button.Text = "Navigate to page two";
+			button.Clicked += Button_Clicked;
+
+			var content = new StackLayout();
+			content.Children.Add(label);
+			content.Children.Add(button);
+
+			Title = "Page one";
+			Content = content;
+		}
+
+		void Button_Clicked(object sender, EventArgs e)
+		{
+			Navigation.PushAsync(new PageTwo());
+		}
+
+		public class PageTwo : ContentPage
+		{
+			public PageTwo()
+			{
+				var label = new Label();
+				label.Text = "Page two...";
+				label.HorizontalTextAlignment = TextAlignment.Center;
+
+				var button = new Button();
+				button.AutomationId = "btn2";
+				button.Text = "Navigate to page three";
+				button.Clicked += Button_Clicked;
+
+				var content = new StackLayout();
+				content.Children.Add(label);
+				content.Children.Add(button);
+
+				Title = "Page two";
+				Content = content;
+			}
+
+			void Button_Clicked(object sender, EventArgs e)
+			{
+				Navigation.PushAsync(new PageThree());
+			}
+		}
+
+		public class PageThree : ContentPage
+		{
+			public PageThree()
+			{
+				var label = new Label();
+				label.Text = "Page three...";
+				label.HorizontalTextAlignment = TextAlignment.Center;
+
+				var button = new Button();
+				button.AutomationId = "btn3";
+				button.Text = "Navigate to page four";
+				button.Clicked += Button_Clicked;
+
+				var content = new StackLayout();
+				content.Children.Add(label);
+				content.Children.Add(button);
+
+				Title = "Page three";
+				Content = content;
+			}
+
+			void Button_Clicked(object sender, EventArgs e)
+			{
+				Navigation.PushAsync(new PageFour());
+			}
+		}
+
+		public class PageFour : ContentPage
+		{
+			public PageFour()
+			{
+				var label = new Label();
+				label.Text = "Last page... Tap back very quick";
+				label.HorizontalTextAlignment = TextAlignment.Center;
+
+				var content = new StackLayout();
+				content.Children.Add(label);
+
+				Title = "Page four";
+				Content = content;
+			}
+		}
+
+#if UITEST
+		[Test]
+		public void Bugzilla38731Test ()
+		{
+			RunningApp.Tap(q => q.Marked("btn1"));
+			RunningApp.Tap(q => q.Marked("btn2"));
+			RunningApp.Tap(q => q.Marked("btn3"));
+			RunningApp.Back();
+			RunningApp.Back();
+			RunningApp.Back();
+		}
+#endif
+	}
+}

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

@@ -550,6 +550,7 @@
       <DependentUpon>Bugzilla54977.xaml</DependentUpon>
     </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla42956.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Bugzilla38731.cs" />
   </ItemGroup>
   <ItemGroup>
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)Bugzilla22229.xaml">

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

@@ -525,7 +525,7 @@ void RemovePage(Page page)
 		void RemoveViewControllers(bool animated)
 		{
 			var controller = TopViewController as ParentingViewController;
-			if (controller == null || controller.Child == null)
+			if (controller == null || controller.Child == null || Platform.GetRenderer(controller.Child) == null)
 				return;
 
 			// Gesture in progress, lets not be proactive and just wait for it to finish