null safety

lib/pages/customer/customer_list.dart

@@ -233,11 +233,11 @@ class _CustomerListState extends State<CustomerList> {
   _share(User user) async {
     MainModel mainModel = Provider.of<MainModel>(context, listen: false);
     String appUrl = mainModel.setting.appUrl;
-    final RenderBox box = context.findRenderObject();
+    final RenderBox? box = context.findRenderObject() as RenderBox;
     await Share.share(
         "Join us on FCS Logistics App. Here is the link:\n $appUrl\n" +
             user.share,
         subject: "Invitation to FCS Logistics App",
-        sharePositionOrigin: box.localToGlobal(Offset.zero) & box.size);
+        sharePositionOrigin: box!.localToGlobal(Offset.zero) & box.size);
   }
 }