[Android] Fix NullPointerException following RemovePage call (#933)

* Add repro for 53179

* Update Android packages to 25+

* Turn off linker to debug

* [Android] Only remove PageContainer < API 25

* Revert "Update Android packages to 25+"

This reverts commit ece252261d709213ad3a3c02c5fd37556bdb093f.

* Revert "Turn off linker to debug"

This reverts commit fdffff71386f1a4b186b89b7421d875e696b7bc7.

* Update test automation

* Add explanatory comment

* Fix test case number

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla53179.cs

@@ -0,0 +1,83 @@
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Bugzilla, 53179, "PopAsync crashing after RemovePage when support packages are updated to 25.1.1", PlatformAffected.Android)]
+	public class Bugzilla53179 : TestNavigationPage
+	{
+		class TestPage : ContentPage
+		{
+			Button nextBtn, rmBtn, popBtn;
+
+			public TestPage(int index)
+			{
+				nextBtn = new Button { Text = "Next Page" };
+				rmBtn = new Button { Text = "Remove previous page" };
+				popBtn = new Button { Text = "Back" };
+
+				nextBtn.Clicked += async (sender, e) => await Navigation.PushAsync(new TestPage(index + 1));
+				rmBtn.Clicked += (sender, e) =>
+				{
+					var stackSize = Navigation.NavigationStack.Count;
+					Navigation.RemovePage(Navigation.NavigationStack[stackSize - 2]);
+					popBtn.IsVisible = true;
+					rmBtn.IsVisible = false;
+				};
+				popBtn.Clicked += async (sender, e) => await Navigation.PopAsync();
+
+				switch (index)
+				{
+					case 3:
+						nextBtn.IsVisible = false;
+						popBtn.IsVisible = false;
+						break;
+					default:
+						rmBtn.IsVisible = false;
+						popBtn.IsVisible = false;
+						break;
+				}
+
+				Content = new StackLayout
+				{
+					Children = {
+					new Label { Text = $"This is page {index}"},
+					nextBtn,
+					rmBtn,
+					popBtn
+				}
+				};
+			}
+		}
+
+
+		protected override void Init()
+		{
+			PushAsync(new TestPage(1));
+		}
+
+#if UITEST
+		[Test]
+		public void Bugzilla53179Test()
+		{
+			RunningApp.WaitForElement(q => q.Marked("Next Page"));
+			RunningApp.Tap(q => q.Marked("Next Page"));
+
+			RunningApp.WaitForElement(q => q.Marked("Next Page"));
+			RunningApp.Tap(q => q.Marked("Next Page"));
+
+			RunningApp.WaitForElement(q => q.Marked("Remove previous page"));
+			RunningApp.Tap(q => q.Marked("Remove previous page"));
+
+			RunningApp.WaitForElement(q => q.Marked("Back"));
+			RunningApp.Tap(q => q.Marked("Back"));
+		}
+#endif
+	}
+}

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

@@ -289,6 +289,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)ListViewNRE.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla55745.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla55365.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Bugzilla53179.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla54036.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)_Template.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue1028.cs" />

Xamarin.Forms.Platform.Android/AppCompat/NavigationPageRenderer.cs

@@ -534,19 +534,27 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
 		void RemovePage(Page page)
 		{
 			IVisualElementRenderer rendererToRemove = Android.Platform.GetRenderer(page);
-			var containerToRemove = (PageContainer)rendererToRemove?.View.Parent;
+
+			if (rendererToRemove != null)
+			{
+				var containerToRemove = (PageContainer)rendererToRemove.View.Parent;
+
+				rendererToRemove.View?.RemoveFromParent();
+
+				rendererToRemove?.Dispose();
+
+				// This causes a NullPointerException in API 25.1+ when we later call ExecutePendingTransactions();
+				// We may want to remove this in the future if it is resolved in the Android SDK.
+				if ((int)Build.VERSION.SdkInt < 25)
+				{
+					containerToRemove?.RemoveFromParent();
+					containerToRemove?.Dispose();
+				}
+			}
 
 			// Also remove this page from the fragmentStack
 			FilterPageFragment(page);
 
-			containerToRemove.RemoveFromParent();
-			if (rendererToRemove != null)
-			{
-				rendererToRemove.View.RemoveFromParent();
-				rendererToRemove.Dispose();
-			}
-			containerToRemove?.Dispose();
-
 			Device.StartTimer(TimeSpan.FromMilliseconds(10), () =>
 			{
 				UpdateToolbar();