Ensure menu children are closed on menu flyout presenter detached

src/Avalonia.Controls/Flyouts/MenuFlyoutPresenter.cs

@@ -43,5 +43,18 @@ namespace Avalonia.Controls
         {
             return new MenuItemContainerGenerator(this);
         }
+
+        protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
+        {
+            base.OnDetachedFromVisualTree(e);
+
+            foreach (var i in LogicalChildren)
+            {
+                if (i is MenuItem menuItem)
+                {
+                    menuItem.IsSubMenuOpen = false;
+                }
+            }
+        }
     }
 }