Use cache when reading declarative format (#16572)

Change-Id: I313ff4e88e13a9c160a4717a40cb13352a30bd6b

server/src/com/vaadin/ui/declarative/DesignAttributeHandler.java

@@ -154,7 +154,7 @@ public class DesignAttributeHandler implements Serializable {
         if (clazz == null) {
             throw new IllegalArgumentException("The clazz can not be null");
         }
-        if (cache.containsKey(clazz.getCanonicalName())) {
+        if (cache.containsKey(clazz)) {
             // NO-OP
             return;
         }
@@ -427,4 +427,4 @@ public class DesignAttributeHandler implements Serializable {
         }
     }
 
-}
+}