Remove controller from UINavigationController stack when page is removed (#10140)

* Remove controller from UINavigationController stack  when page is removed

* - add uitest

* - remove extra code

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue6484.cs

@@ -0,0 +1,74 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Text;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+using System.Threading.Tasks;
+
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+using Xamarin.Forms.Core.UITests;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 6484, "[iOS] Shell - Go back two pages crashes the app with a NullReferenceException",
+		PlatformAffected.iOS)]
+#if UITEST
+	[NUnit.Framework.Category(UITestCategories.Shell)]
+#endif
+	public class Issue6484 : TestShell
+	{
+		StackLayout layout = null;
+		ContentPage page = null;
+		protected override void Init()
+		{
+			layout = new StackLayout();
+			page = new ContentPage()
+			{
+				Content = layout
+			};
+
+			page.Appearing += OnPageAppearing;
+
+			AddContentPage(page);
+
+		}
+
+		async void OnPageAppearing(object sender, EventArgs e)
+		{
+			page.Appearing -= OnPageAppearing;
+
+			var removeMe = new ContentPage();
+			await Navigation.PushAsync(removeMe);
+			await Navigation.PushAsync(new ContentPage());
+
+			await Task.Delay(1);
+
+			Navigation.RemovePage(removeMe);
+			await Navigation.PopAsync();
+
+
+			await Task.Delay(1);
+			layout.Children.Add(
+				new Label()
+				{
+					Text = "If app hasn't crashed test has succeeded",
+					AutomationId = "Success"
+				});
+
+		}
+
+#if UITEST
+		[Test]
+		public void RemovingIntermediatePagesBreaksShell()
+		{
+			RunningApp.WaitForElement("Success");
+		}
+#endif
+	}
+}

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

@@ -10,6 +10,7 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(MSBuildThisFileDirectory)CollectionViewGroupTypeIssue.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue6484.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue6187.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue3228.xaml.cs">
       <DependentUpon>Issue3228.xaml</DependentUpon>
@@ -1460,12 +1461,12 @@
       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)Issue9735.xaml">
-       <SubType>Designer</SubType>
-       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)Issue9305.xaml">
-       <SubType>Designer</SubType>
-       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)Issue9588.xaml">
       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>

Xamarin.Forms.Core/Shell/ShellSection.cs

@@ -724,20 +724,22 @@ namespace Xamarin.Forms
 			bool currentPage = (((IShellSectionController)this).PresentedPage) == page;
 			var stack = _navStack.ToList();
 			stack.Remove(page);
-			var allow = ((IShellController)Shell).ProposeNavigation(
-				ShellNavigationSource.Remove,
-				ShellItem,
-				this,
-				CurrentItem,
-				stack,
-				true
-			);
+			var allow = (!currentPage) ? true : 
+				((IShellController)Shell).ProposeNavigation(
+					ShellNavigationSource.Remove,
+					ShellItem,
+					this,
+					CurrentItem,
+					stack,
+					true
+				);
 
 			if (!allow)
 				return;
 
 			if(currentPage)
 				PresentedPageDisappearing();
+
 			_navStack.Remove(page);
 
 			if(currentPage)

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

@@ -336,15 +336,20 @@ namespace Xamarin.Forms.Platform.iOS
 			var page = e.Page;
 
 			var renderer = Platform.GetRenderer(page);
+			var viewController = renderer?.ViewController;
 
-			if (renderer != null)
+			if (viewController == null && _trackers.ContainsKey(page))
+				viewController = _trackers[page].ViewController;
+
+			if (viewController != null)
 			{
-				if (renderer.ViewController == TopViewController)
+				if (viewController == TopViewController)
 				{
 					e.Animated = false;
 					OnPopRequested(e);
 				}
-				ViewControllers = ViewControllers.Remove(renderer.ViewController);
+
+				ViewControllers = ViewControllers.Remove(viewController);
 				DisposePage(page);
 			}
 		}
@@ -371,10 +376,14 @@ namespace Xamarin.Forms.Platform.iOS
 		{
 			if (_trackers.TryGetValue(page, out var tracker))
 			{
+				if(tracker.ViewController != null && ViewControllers.Contains(tracker.ViewController))
+					ViewControllers = ViewControllers.Remove(_trackers[page].ViewController);
+
 				tracker.Dispose();
 				_trackers.Remove(page);
 			}
 
+
 			var renderer = Platform.GetRenderer(page);
 			if (renderer != null)
 			{