[Android] Fix NavigationPage dispose crash when it parents a MasterDetailPage (#577)

* fix navigation page dispose crash

* changes after review

diff --git a/Xamarin.Forms.Platform.Android/AppCompat/NavigationPageRenderer.cs b/Xamarin.Forms.Platform.Android/AppCompat/NavigationPageRenderer.cs
index 518e983..d22d6a2 100644 (file)
--- a/Xamarin.Forms.Platform.Android/AppCompat/NavigationPageRenderer.cs
+++ b/Xamarin.Forms.Platform.Android/AppCompat/NavigationPageRenderer.cs
@@ -39,6 +39,8 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
                int _lastActionBarHeight = -1;
                AToolbar _toolbar;
                ToolbarTracker _toolbarTracker;
+               DrawerMultiplexedListener _drawerListener;
+               DrawerLayout _drawerLayout;
                bool _toolbarVisible;
 
                // The following is based on https://android.googlesource.com/platform/frameworks/support/+/refs/heads/master/v4/java/android/support/v4/app/FragmentManager.java#849
@@ -133,29 +135,6 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
                                        }
                                }
 
-                               if (Element != null)
-                               {
-                                       foreach (Element element in PageController.InternalChildren)
-                                       {
-                                               var child = element as VisualElement;
-                                               if (child == null)
-                                               {
-                                                       continue;
-                                               }
-
-                                               IVisualElementRenderer renderer = Android.Platform.GetRenderer(child);
-                                               renderer?.Dispose();
-                                       }
-
-                                       var navController = (INavigationPageController)Element;
-
-                                       navController.PushRequested -= OnPushed;
-                                       navController.PopRequested -= OnPopped;
-                                       navController.PopToRootRequested -= OnPoppedToRoot;
-                                       navController.InsertPageBeforeRequested -= OnInsertPageBeforeRequested;
-                                       navController.RemovePageRequested -= OnRemovePageRequested;
-                               }
-
                                if (_toolbarTracker != null)
                                {
                                        _toolbarTracker.CollectionChanged -= ToolbarTrackerOnCollectionChanged;
@@ -173,13 +152,51 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
                                if (_drawerLayout != null && _drawerListener != null)
                                {
                                        _drawerLayout.RemoveDrawerListener(_drawerListener);
+                               }
+
+                               if (_drawerListener != null)
+                               {
+                                       _drawerListener.Dispose();
                                        _drawerListener = null;
                                }
 
-                               _drawerToggle = null;
+                               if (_drawerToggle != null)
+                               {
+                                       _drawerToggle.Dispose();
+                                       _drawerToggle = null;
+                               }
+
+                               if (_backgroundDrawable != null)
+                               {
+                                       _backgroundDrawable.Dispose();
+                                       _backgroundDrawable = null;
+                               }
 
                                Current = null;
 
+                               // We dispose the child renderers after cleaning up everything related to DrawerLayout in case
+                               // one of the children is a MasterDetailPage (which may dispose of the DrawerLayout).
+                               if (Element != null)
+                               {
+                                       foreach (Element element in PageController.InternalChildren)
+                                       {
+                                               var child = element as VisualElement;
+                                               if (child == null)
+                                                       continue;
+
+                                               IVisualElementRenderer renderer = Android.Platform.GetRenderer(child);
+                                               renderer?.Dispose();
+                                       }
+
+                                       var navController = (INavigationPageController)Element;
+
+                                       navController.PushRequested -= OnPushed;
+                                       navController.PopRequested -= OnPopped;
+                                       navController.PopToRootRequested -= OnPoppedToRoot;
+                                       navController.InsertPageBeforeRequested -= OnInsertPageBeforeRequested;
+                                       navController.RemovePageRequested -= OnRemovePageRequested;
+                               }
+
                                Device.Info.PropertyChanged -= DeviceInfoPropertyChanged;
                        }
 
@@ -465,9 +482,6 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
                        RemovePage(e.Page);
                }
 
-               private DrawerMultiplexedListener _drawerListener;
-               private DrawerLayout _drawerLayout;
-
                void RegisterToolbar()
                {
                        Context context = Context;
@@ -499,7 +513,7 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
                        if (renderer == null)
                                return;
 
-                       _drawerLayout = (DrawerLayout)renderer;
+                       _drawerLayout = renderer;
                        _drawerToggle = new ActionBarDrawerToggle((Activity)context, _drawerLayout, bar, global::Android.Resource.String.Ok, global::Android.Resource.String.Ok)
                        {
                                ToolbarNavigationClickListener = new ClickListener(Element)