Fix PlatformMenuItems with onSelectedIntent are never enabled (flutter#121885)

Fix PlatformMenuItems with onSelectedIntent are never enabled

Author: jmatth

packages/flutter/lib/src/widgets/platform_menu_bar.dart

@@ -738,7 +738,8 @@ class PlatformMenuItem with Diagnosticable {
   /// An optional callback that is called when this [PlatformMenuItem] is
   /// selected.
   ///
-  /// If unset, this menu item will be disabled.
+  /// At most one of [onSelected] and [onSelectedIntent] may be set. If neither
+  /// field is set, this menu item will be disabled.
   final VoidCallback? onSelected;
 
   /// Returns a callback, if any, to be invoked if the platform menu receives a
@@ -760,7 +761,8 @@ class PlatformMenuItem with Diagnosticable {
   /// An optional intent that is invoked when this [PlatformMenuItem] is
   /// selected.
   ///
-  /// If unset, this menu item will be disabled.
+  /// At most one of [onSelected] and [onSelectedIntent] may be set. If neither
+  /// field is set, this menu item will be disabled.
   final Intent? onSelectedIntent;
 
   /// Returns all descendant [PlatformMenuItem]s of this item.
@@ -805,7 +807,7 @@ class PlatformMenuItem with Diagnosticable {
     return <String, Object?>{
       _kIdKey: getId(item),
       _kLabelKey: item.label,
-      _kEnabledKey: item.onSelected != null,
+      _kEnabledKey: item.onSelected != null || item.onSelectedIntent != null,
       if (shortcut != null)...shortcut.serializeForMenu().toChannelRepresentation(),
     };
   }