phase3: bulk null-safety fixes

- validator: (String value) -> (String? value) [30 files]
- ElevatedButton.styleFrom(primary:) -> backgroundColor: [31 files]
- global Key? param fix [129 files]

lib/widgets/desktop/desktop_order_detail.dart

@@ -1177,7 +1177,7 @@ class DesktopOrderDetailState extends State<DesktopOrderDetail> {
                 S.of(context).no,
               ),
               style: TextButton.styleFrom(
-                primary: Theme.of(context).primaryColor,
+                backgroundColor: Theme.of(context).primaryColor,
               ),
               onPressed: () {
                 Navigator.of(context).pop();