Cleanup for API error handling

Ref: https://stackoverflow.com/questions/54617432/looking-up-a-deactivated-widgets-ancestor-is-unsafe

lib/widget/part_detail.dart

@@ -217,7 +217,7 @@ class _PartDisplayState extends RefreshableState<PartDetailWidget> {
       ListTile(
         title: Text(I18N.of(context).stock),
         leading: FaIcon(FontAwesomeIcons.boxes),
-        trailing: Text("${part.inStock}"),
+        trailing: Text("${part.inStockString}"),
         onTap: () {
           _showStock(context);
         },