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_orders.dart

@@ -263,7 +263,7 @@ class DesktopOrdersState extends State<DesktopOrders> with SingleTickerProviderS
                 ),
               ),
               style: ElevatedButton.styleFrom(
-                primary: Colors.redAccent,
+                backgroundColor: Colors.redAccent,
               ),
               onPressed: () {
                 Routes.router.navigateTo(context, '/paynow/${order.id}');
@@ -278,7 +278,7 @@ class DesktopOrdersState extends State<DesktopOrders> with SingleTickerProviderS
                 ),
               ),
               style: ElevatedButton.styleFrom(
-                primary: Theme.of(context).primaryColor,
+                backgroundColor: Theme.of(context).primaryColor,
               ),
               onPressed: () {
                 Utils.orderAgain(context, order.cartInfo);
@@ -295,7 +295,7 @@ class DesktopOrdersState extends State<DesktopOrders> with SingleTickerProviderS
               ),
             ),
             style: ElevatedButton.styleFrom(
-              primary: Theme.of(context).primaryColor,
+              backgroundColor: Theme.of(context).primaryColor,
             ),
             onPressed: () {
               Utils.orderAgain(context, order.cartInfo);