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/general/add_remove_button.dart

@@ -125,7 +125,7 @@ class AddRemoveButtonState extends State<AddRemoveButton> {
           badgeColor: Colors.lightBlueAccent,
           child: ElevatedButton.icon(
             style: ElevatedButton.styleFrom(
-              primary: Colors.redAccent,
+              backgroundColor: Colors.redAccent,
               onPrimary: Colors.white,
               padding: EdgeInsets.only(left: 32, right: 32, top: 20, bottom: 20),
               elevation: 2.0,
@@ -154,7 +154,7 @@ class AddRemoveButtonState extends State<AddRemoveButton> {
       } else {
         return ElevatedButton.icon(
           style: ElevatedButton.styleFrom(
-            primary: Colors.redAccent,
+            backgroundColor: Colors.redAccent,
             padding: EdgeInsets.only(left: 32, right: 32, top: 20, bottom: 20),
             elevation: 2.0,
             shape: new RoundedRectangleBorder(

lib/widgets/general/download_item.dart

@@ -244,7 +244,7 @@ class DownloadItemState extends State<DownloadItem> {
     } else if (downloadUrl != null && downloadUrl.isNotEmpty) {
       return ElevatedButton(
         style: ElevatedButton.styleFrom(
-          primary: Theme.of(context).primaryColor,
+          backgroundColor: Theme.of(context).primaryColor,
           padding: EdgeInsets.all(8)
         ),
         child: Row(