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/mobile/create_online_store_1.dart

@@ -230,7 +230,7 @@ class CreateOnlineStore1State extends State<CreateOnlineStore1> {
                         fontSize: 18.0
                     ),
                     autofocus: true,
-                    validator: (String value) {
+                    validator: (String? value) {
                       if (value.trim().isEmpty) {
                         return S.of(context).domains_separated_comma;
                       }