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/desktop/desktop_new_comment.dart

@@ -175,7 +175,7 @@ class DesktopNewCommentState extends State<DesktopNewComment> {
                       ),
                     ),
                     style: ElevatedButton.styleFrom(
-                      primary: Theme.of(context).primaryColor,
+                      backgroundColor: Theme.of(context).primaryColor,
                     ),
                     onPressed: () {
                       _submit();
@@ -246,7 +246,7 @@ class DesktopNewCommentState extends State<DesktopNewComment> {
                               Navigator.of(context).pop();
                             },
                             style: TextButton.styleFrom(
-                              primary: Theme.of(context).primaryColor,
+                              backgroundColor: Theme.of(context).primaryColor,
                             ),
                           ),
                           TextButton(