Stringify URL when not found (#1665)

diff --git a/dwds/lib/src/loaders/legacy.dart b/dwds/lib/src/loaders/legacy.dart
index 02e0148..87308c1 100644
--- a/dwds/lib/src/loaders/legacy.dart
+++ b/dwds/lib/src/loaders/legacy.dart
@@ -72,7 +72,7 @@
   ) : super(assetReader);
 
   @override
-  Handler get handler => (request) => Response.notFound(request.url);
+  Handler get handler => (request) => Response.notFound(request.url.toString());
 
   @override
   String get id => 'legacy';