FEATURE: implement loadExtraData as empty function in Handler

app/Handlers/Handler.php

@@ -56,6 +56,10 @@ abstract class Handler extends BaseHandler implements AppHandler
         // temporarily nothing to do
     }
 
+    public static function loadExtraData(Content $content) {
+        // temporarily nothing to do
+    }
+
     public static function getExtendedFormInputsEdit()
     {
         return self::getExtendedFormInputsCreate();