Menu interaction handler fixes.

src/Avalonia.Controls/MenuItem.cs

@@ -101,7 +101,7 @@ namespace Avalonia.Controls
 
         private ICommand? _command;
         private bool _commandCanExecute = true;
-        private Popup _popup;
+        private Popup? _popup;
 
         /// <summary>
         /// Initializes static members of the <see cref="MenuItem"/> class.
@@ -145,7 +145,7 @@ namespace Avalonia.Controls
                 {
                     var parent = x as Control;
                     return parent?.GetObservable(DefinitionBase.PrivateSharedSizeScopeProperty) ??
-                        Observable.Return<DefinitionBase.SharedSizeScope>(null);
+                        Observable.Return<DefinitionBase.SharedSizeScope?>(null);
                 });
 
             this.Bind(DefinitionBase.PrivateSharedSizeScopeProperty, parentSharedSizeScope);

src/Avalonia.Controls/Platform/DefaultMenuInteractionHandler.cs

@@ -148,6 +148,7 @@ namespace Avalonia.Controls.Platform
             {
                 case Key.Up:
                 case Key.Down:
+                {
                     if (item?.IsTopLevel == true)
                     {
                         if (item.HasSubMenu && !item.IsSubMenuOpen)
@@ -161,8 +162,10 @@ namespace Avalonia.Controls.Platform
                         goto default;
                     }
                     break;
+                }
 
                 case Key.Left:
+                {
                     if (item?.Parent is IMenuItem parent && !parent.IsTopLevel && parent.IsSubMenuOpen)
                     {
                         parent.Close();
@@ -174,8 +177,10 @@ namespace Avalonia.Controls.Platform
                         goto default;
                     }
                     break;
+                }
 
                 case Key.Right:
+                {
                     if (item != null && !item.IsTopLevel && item.HasSubMenu)
                     {
                         Open(item, true);
@@ -186,8 +191,10 @@ namespace Avalonia.Controls.Platform
                         goto default;
                     }
                     break;
+                }
 
                 case Key.Enter:
+                {
                     if (item != null)
                     {
                         if (!item.HasSubMenu)
@@ -202,12 +209,14 @@ namespace Avalonia.Controls.Platform
                         e.Handled = true;
                     }
                     break;
+                }
 
                 case Key.Escape:
-                    if (item?.Parent != null)
+                {
+                    if (item?.Parent is IMenuElement parent)
                     {
-                        item.Parent.Close();
-                        item.Parent.Focus();
+                        parent.Close();
+                        parent.Focus();
                     }
                     else
                     {
@@ -216,8 +225,10 @@ namespace Avalonia.Controls.Platform
 
                     e.Handled = true;
                     break;
+                }
 
                 default:
+                {
                     var direction = e.Key.ToNavigationDirection();
 
                     if (direction.HasValue)
@@ -246,6 +257,7 @@ namespace Avalonia.Controls.Platform
                     }
 
                     break;
+                }
             }
 
             if (!e.Handled && item?.Parent is IMenuItem parentItem)