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_view_ticket.dart

@@ -143,7 +143,7 @@ class DesktopViewTicketState extends State<DesktopViewTicket> {
                   fontSize: 18.0
               ),
               autofocus: false,
-              validator: (String value) {
+              validator: (String? value) {
                 if (value.trim().isEmpty) {
                   return S.of(context).this_field_is_required;
                 }
@@ -194,7 +194,7 @@ class DesktopViewTicketState extends State<DesktopViewTicket> {
         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).reply,
@@ -644,7 +644,7 @@ class DesktopViewTicketState extends State<DesktopViewTicket> {
             ElevatedButton(
               child: Text(S.of(context).yes_i_am_sure),
               style: ElevatedButton.styleFrom(
-                primary: Theme.of(context).primaryColor,
+                backgroundColor: Theme.of(context).primaryColor,
               ),
               onPressed: () {
                 Navigator.of(context).pop();