Check if DataObject before calling ORM/::get() on class

A shortcode may have an id attribute without necessarily representing a database record, hence we should check first.

src/Controller/ShortcodableController.php

@@ -229,7 +229,7 @@ public function shortcodePlaceHolder($request)
             return;
         }
 
-        if ($id) {
+        if ($id && is_subclass_of($classname, 'DataObject')) {
             $object = $classname::get()->byID($id);
         } else {
             $object = singleton($classname);