NavigationView: Fix crash when launched in Top mode with hierarchical navigation (microsoft/microsoft-ui-xaml#3166)

Author: Kinnara

ModernWpf.Controls/NavigationView/NavigationViewItem.cs

@@ -516,7 +516,9 @@ void UpdateVisualStateForChevron()
 
         internal bool HasChildren()
         {
-            return MenuItems.Count > 0 || (MenuItemsSource != null && m_repeater.ItemsSourceView.Count > 0) || HasUnrealizedChildren;
+            return MenuItems.Count > 0
+                || (MenuItemsSource != null && m_repeater != null && m_repeater.ItemsSourceView.Count > 0)
+                || HasUnrealizedChildren;
         }
 
         bool ShouldShowIcon()

test/ModernWpfTestApp/ApiTests/NavigationViewTests.cs

@@ -846,7 +846,7 @@ 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(true);
@@ -855,11 +855,11 @@ public void VerifyMenuItemAndContainerMappingMenuItemsSource()
                 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.");
 
@@ -1080,5 +1080,23 @@ public void VerifyClearingItemsCollectionDoesNotCrashWhenItemSelectedOnTopNav()
                 itemsSource.Clear();
             });
         }
+
+        [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();
+            });
+        }
     }
 }