Verify FragmentContainer created when queueing fragment transaction (Android) (#865)

* Add check for disposed MasterDetailContainer when queueing fragment transaction

* Fix various ObjectDisposedExceptions when setting main page

* Add check for legacy renderers when querying control properties

2 files changed, 31 insertions, 4 deletions

diff --git a/Xamarin.Forms.Platform.Android/AppCompat/MasterDetailContainer.cs b/Xamarin.Forms.Platform.Android/AppCompat/MasterDetailContainer.cs
index 0265cd3d..50840926 100644
--- a/Xamarin.Forms.Platform.Android/AppCompat/MasterDetailContainer.cs
+++ b/Xamarin.Forms.Platform.Android/AppCompat/MasterDetailContainer.cs
@@ -1,3 +1,4 @@
+using System;
 using Android.App;
 using Android.Content;
 using Android.OS;
@@ -86,7 +87,7 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
 				// The renderers for NavigationPage and TabbedPage both host fragments, so they need to be wrapped in a 
 				// FragmentContainer in order to get isolated fragment management
 				Fragment fragment = FragmentContainer.CreateInstance(page);
-
+				
 				var fc = fragment as FragmentContainer;
 
 				fc?.SetOnCreateCallback(pc =>
@@ -112,7 +113,17 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
 
 				_currentFragment = fragment;
 
-				new Handler(Looper.MainLooper).PostAtFrontOfQueue(() => FragmentManager.ExecutePendingTransactions());
+				new Handler(Looper.MainLooper).PostAtFrontOfQueue(() =>
+				{
+					if (_pageContainer == null)
+					{
+						// The view we're hosting in the fragment was never created (possibly we're already 
+						// navigating to another page?) so there's nothing to commit
+						return;
+					}
+
+					FragmentManager.ExecutePendingTransactions();
+				});
 			}
 		}
 
diff --git a/Xamarin.Forms.Platform.Android/AppCompat/Platform.cs b/Xamarin.Forms.Platform.Android/AppCompat/Platform.cs
index 0416ae3a..872bac77 100644
--- a/Xamarin.Forms.Platform.Android/AppCompat/Platform.cs
+++ b/Xamarin.Forms.Platform.Android/AppCompat/Platform.cs
@@ -3,10 +3,12 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
 using Android.Content;
+using Android.OS;
 using Android.Views;
 using Android.Views.Animations;
 using ARelativeLayout = Android.Widget.RelativeLayout;
 using Xamarin.Forms.Internals;
+using Debug = System.Diagnostics.Debug;
 
 namespace Xamarin.Forms.Platform.Android.AppCompat
 {
@@ -219,12 +221,14 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
 		internal void SetPage(Page newRoot)
 		{
 			var layout = false;
+			List<IVisualElementRenderer> toDispose = null;
+
 			if (Page != null)
 			{
 				_renderer.RemoveAllViews();
 
-				foreach (IVisualElementRenderer rootRenderer in _navModel.Roots.Select(Android.Platform.GetRenderer))
-					rootRenderer.Dispose();
+				toDispose = _navModel.Roots.Select(Android.Platform.GetRenderer).ToList();
+
 				_navModel = new NavigationModel();
 
 				layout = true;
@@ -240,6 +244,18 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
 			AddChild(Page, layout);
 
 			Application.Current.NavigationProxy.Inner = this;
+
+			if (toDispose?.Count > 0)
+			{
+				// Queue up disposal of the previous renderers after the current layout updates have finished
+				new Handler(Looper.MainLooper).Post(() =>
+				{	
+					foreach (IVisualElementRenderer rootRenderer in toDispose)
+					{
+						rootRenderer.Dispose();
+					}
+				});
+			}
 		}
 
 		void AddChild(Page page, bool layout = false)