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_comment.dart

@@ -210,7 +210,7 @@ class MobileNewCommentState extends State<MobileNewComment> {
                               Navigator.of(context).pop();
                             },
                             style: TextButton.styleFrom(
-                              primary: Theme.of(context).primaryColor,
+                              backgroundColor: Theme.of(context).primaryColor,
                             ),
                           ),
                           TextButton(