XWIKI-7670: Make class references always local

Fix potential classcast exception

xwiki-platform-core/xwiki-platform-oldcore/src/main/java/com/xpn/xwiki/objects/BaseCollection.java

@@ -192,7 +192,7 @@ public void setXClassReference(EntityReference xClassReference)
         EntityReference ref = xClassReference;
 
         if (ref != null) {
-            WikiReference wiki = (WikiReference) xClassReference.extractReference(EntityType.WIKI);
+            EntityReference wiki = xClassReference.extractReference(EntityType.WIKI);
             if (wiki != null) {
                 ref = xClassReference.removeParent(wiki);