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_login.dart

@@ -103,7 +103,7 @@ class MobileLoginState extends State<MobileLogin> {
                       fontSize: 18.0
                   ),
                   autofocus: true,
-                  validator: (String value) {
+                  validator: (String? value) {
                     if (value.trim().isEmpty) {
                       return S.of(context).this_field_is_required;
                     }
@@ -143,7 +143,7 @@ class MobileLoginState extends State<MobileLogin> {
                       fontSize: 18.0
                   ),
                   obscureText: passwordVisible,
-                  validator: (String value) {
+                  validator: (String? value) {
                     if (value.trim().isEmpty) {
                       return S.of(context).password_is_required;
                     }
@@ -196,7 +196,7 @@ class MobileLoginState extends State<MobileLogin> {
             padding: EdgeInsets.only(top: 20.0, left: 16.0, right: 16.0, bottom: 20.0),
             child: ElevatedButton(
               style: ElevatedButton.styleFrom(
-                primary: Theme.of(context).primaryColor,
+                backgroundColor: Theme.of(context).primaryColor,
               ),
               child: Text(
                 S.of(context).login,