fix: #383

Author: w568w

lib/widget/forum/auto_banner.dart

@@ -153,10 +153,14 @@ class AutoBannerState extends State<AutoBanner> {
             return Container();
           }
           list ??= AnnouncementRepository.getInstance().getBannerExtras();
-          // Only update if list is null
+          // Only update if list is null. And if the update fails, return nil
           if (list == null && !updateBannerList()) {
             return Container();
           }
+          // If the list is empty (i.e. no banner), return nil
+          if (list!.isEmpty) {
+            return Container(); // FIXME: using `nil` here will break the layout. Don't know why.
+          }
 
           // Since the banner is not a fixed size, we need to use [SizedByChildBuilder]
           // to get the height of the banner. Otherwise, [Swiper] will have infinite