phase3: targeted local-var nullable (strict type) + nullfix cascade. 379->309

lib/widgets/general/download_item.dart

@@ -186,9 +186,9 @@ class DownloadItemState extends State<DownloadItem> {
   }
 
   Widget getDownloadButton() {
-    String downloadUrl;
+    String? downloadUrl;
     String os = Utils.getOs();
-    String selectedOs;
+    String? selectedOs;
     List<String> supportedOss = [];
     for (int i = 0; i < (widget.desc['urls'] as List).length; i++) {
       supportedOss.add((widget.desc['urls'] as List)[i]['os']);
@@ -255,7 +255,7 @@ class DownloadItemState extends State<DownloadItem> {
               padding: EdgeInsets.only(right: 0.0, left: 6.0, top: 6.0, bottom: 6.0),
               child: Icon(
                 IconData(
-                    Utils.getOsFontHex(selectedOs),
+                    Utils.getOsFontHex(selectedOs!),
                     fontFamily: 'wisetronic',
                     fontPackage: null
                 ),