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_checkout.dart

@@ -127,7 +127,7 @@ class DesktopCheckoutState extends State<DesktopCheckout> with SingleTickerProvi
                 ),
                 ElevatedButton(
                   style: ElevatedButton.styleFrom(
-                    primary: Theme.of(context).primaryColor,
+                    backgroundColor: Theme.of(context).primaryColor,
                   ),
                   child: Text(
                     S.of(context).ok,
@@ -1652,7 +1652,7 @@ class DesktopCheckoutState extends State<DesktopCheckout> with SingleTickerProvi
                   ),
                   labelText: S.of(context).enter_coupon_code,
                 ),
-                validator: (String value) {
+                validator: (String? value) {
                   return null;
                 },
               ),