action_sheet: Fix use of stale context

On hot reload, the `DesignVariables.of(context)` lookup in this
builder could throw because `context` was no longer mounted.
(I'm not sure if the issue could also be triggered in release builds.)

lib/widgets/action_sheet.dart

@@ -64,7 +64,7 @@ void showMessageActionSheet({required BuildContext context, required Message mes
     clipBehavior: Clip.antiAlias,
     useSafeArea: true,
     isScrollControlled: true,
-    builder: (BuildContext _) {
+    builder: (BuildContext context) {
       return SafeArea(
         minimum: const EdgeInsets.only(bottom: 16),
         child: Padding(