NavigationView: Fix crash when launched in Top mode with hierarchical navigation

dev/NavigationView/NavigationViewItem.cpp

@@ -492,7 +492,9 @@ void NavigationViewItem::UpdateVisualStateForChevron()
 
 bool NavigationViewItem::HasChildren()
 {
-    return MenuItems().Size() > 0 || (MenuItemsSource() != nullptr && m_repeater.get().ItemsSourceView().Count() > 0) || HasUnrealizedChildren();
+    return MenuItems().Size() > 0
+        || (MenuItemsSource() != nullptr && m_repeater != nullptr && m_repeater.get().ItemsSourceView().Count() > 0)
+        || HasUnrealizedChildren();
 }
 
 bool NavigationViewItem::ShouldShowIcon()

dev/NavigationView/NavigationView_ApiTests/NavigationViewTests.cs

@@ -681,19 +681,19 @@ public void VerifyMenuItemAndContainerMappingMenuItemsSource()
                 var navView = new NavigationView();
                 MUXControlsTestApp.App.TestContentRoot = navView;
 
-                navView.MenuItemsSource = new ObservableCollection<String> { "Item 1", "Item 2" }; ;
+                navView.MenuItemsSource = new ObservableCollection<string> { "Item 1", "Item 2" };
                 navView.Width = 1008; // forces the control into Expanded mode so that the menu renders
 
                 MUXControlsTestApp.App.TestContentRoot.UpdateLayout();
 
                 var menuItem = "Item 2";
                 // Get container for item
                 var itemContainer = navView.ContainerFromMenuItem(menuItem) as NavigationViewItem;
-                bool correctContainerReturned = itemContainer != null && (itemContainer.Content as String) == menuItem;
+                bool correctContainerReturned = itemContainer != null && (itemContainer.Content as string) == menuItem;
                 Verify.IsTrue(correctContainerReturned, "Correct container should be returned for passed in menu item.");
 
                 // Get item for container
-                var returnedItem = navView.MenuItemFromContainer(itemContainer) as String;
+                var returnedItem = navView.MenuItemFromContainer(itemContainer) as string;
                 bool correctItemReturned = returnedItem != null && returnedItem == menuItem;
                 Verify.IsTrue(correctItemReturned, "Correct item should be returned for passed in container.");
 
@@ -872,5 +872,26 @@ public void VerifyOverflowButtonToolTip()
                 Verify.IsTrue(testCondition, "ToolTip text should have been \"More\".");
             });
         }
+
+        [TestMethod]
+        public void VerifyHierarchicalNavigationTopModeMenuItemsSourceDoesNotCrash()
+        {
+            RunOnUIThread.Execute(() =>
+            {
+                var navView = new NavigationView();
+                Content = navView;
+
+                navView.PaneDisplayMode = NavigationViewPaneDisplayMode.Top;
+
+                var childItem = new NavigationViewItem() { Content = "Item 1.1" };
+                var parentItem = new NavigationViewItem() { Content = "Item 1", MenuItemsSource = new ObservableCollection<NavigationViewItem>() { childItem } };
+                navView.MenuItemsSource = new ObservableCollection<NavigationViewItem>() { parentItem };             
+
+                Content.UpdateLayout();
+
+                // If we don't get here, app has crashed. This verify signals the NavigationView was launched in Top mode without crashing.
+                Verify.IsTrue(true);
+            });
+        }
     }
 }