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/mobile_new_user.dart

@@ -87,7 +87,7 @@ class MobileNewUserState extends State<MobileNewUser> {
                         fontSize: 18.0
                     ),
                     autofocus: true,
-                    validator: (String value) {
+                    validator: (String? value) {
                       if (value.trim().isEmpty) {
                         return S.of(context).mobile_or_email_is_required;
                       }
@@ -170,7 +170,7 @@ class MobileNewUserState extends State<MobileNewUser> {
                     style: TextStyle(
                         fontSize: 18.0
                     ),
-                    validator: (String value) {
+                    validator: (String? value) {
                       if (value.trim().isEmpty) {
                         return S.of(context).verification_code_is_required;
                       }
@@ -203,7 +203,7 @@ class MobileNewUserState extends State<MobileNewUser> {
             alignment: Alignment.centerRight,
             child: ElevatedButton(
               style: ElevatedButton.styleFrom(
-                primary: Theme.of(context).primaryColor,
+                backgroundColor: Theme.of(context).primaryColor,
               ),
               child: Text(
                 S.of(context).register,