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_me.dart

@@ -687,7 +687,7 @@ class MobileMeState extends State<MobileMe> {
                                       ),
                                       ElevatedButton(
                                         style: ElevatedButton.styleFrom(
-                                          primary: Theme.of(context).primaryColor,
+                                          backgroundColor: Theme.of(context).primaryColor,
                                           textStyle: TextStyle(
                                             color: Colors.white,
                                           ),