somewhat working updating menus.

src/Avalonia.Native/AvaloniaNativeMenuExporter.cs

@@ -105,22 +105,30 @@ namespace Avalonia.Native
             if (appMenu is null)
             {
                 appMenu = _factory.CreateMenu();
-
-                _factory.SetAppMenu(appMenu);
             }
 
+            
             var menuItem = menu.Parent;
 
+            var appMenuHolder = menuItem?.Parent;
+
             if (menu.Parent is null)
             {
                 menuItem = new NativeMenuItem();
+            }
 
-                menuItem.Parent = new NativeMenu();
+            if(appMenuHolder is null)
+            {
+                appMenuHolder = new NativeMenu();
+
+                appMenuHolder.Add(menuItem);
             }
 
             menuItem.Menu = menu;
 
-            appMenu.Update(this, _factory, menuItem.Parent);
+            appMenu.Update(this, _factory, appMenuHolder);
+
+            _factory.SetAppMenu(appMenu);
         }
 
         private void SetMenu(IAvnWindow avnWindow, NativeMenu menu)
@@ -130,10 +138,12 @@ namespace Avalonia.Native
             if (appMenu is null)
             {
                 appMenu = _factory.CreateMenu();
-                avnWindow.SetMainMenu(appMenu);
+                
             }
 
             appMenu.Update(this, _factory, menu);
+
+            avnWindow.SetMainMenu(appMenu);
         }
     }
 }

src/Avalonia.Native/IAvnAppMenuItem.cs

@@ -18,9 +18,12 @@ namespace Avalonia.Native.Interop
 
             Managed.PropertyChanged += Item_PropertyChanged;
 
-            using (var buffer = new Utf8Buffer(item.Header))
+            if(!string.IsNullOrWhiteSpace(item.Header))
             {
-                Title = buffer.DangerousGetHandle();
+                using (var buffer = new Utf8Buffer(item.Header))
+                {
+                    Title = buffer.DangerousGetHandle();
+                }
             }
 
             if (item.Gesture != null)
@@ -48,7 +51,8 @@ namespace Avalonia.Native.Interop
                     _subMenu = factory.CreateMenu();
                 }
 
-                _subMenu.Update(exporter, factory, item.Menu);
+                SetSubMenu(_subMenu);
+                _subMenu.Update(exporter, factory, item.Menu, item.Header);
             }
 
             if (item.Menu == null && _subMenu != null)