fix: assertion when scrollController is not attached

@@ -231,7 +231,8 @@ class _ModalBottomSheetState extends State<ModalBottomSheet>
   void _handleScrollUpdate(ScrollNotification notification) {
     if (notification.metrics.pixels <= notification.metrics.minScrollExtent) {
       //Check if listener is same from scrollController
-      if (_scrollController.position.pixels != notification.metrics.pixels) {
+      if (_scrollController.hasClients &&
+          _scrollController.position.pixels != notification.metrics.pixels) {
         return;
       }
       DragUpdateDetails dragDetails;