Fix impersonating

src/Admin/UserAdmin.php

@@ -55,7 +55,7 @@ class UserAdmin extends SonataUserAdmin
 
         if ('dev' == $_ENV['APP_ENV'] && $this->isGranted('ROLE_ALLOWED_TO_SWITCH')) {
             $listMapper
-                ->addIdentifier('impersonating', 'string', ['template' => '@SonataUser/Admin/Field/impersonating.html.twig'])
+                ->add('impersonating', 'string', ['template' => '@SonataUser/Admin/Field/impersonating.html.twig'])
             ;
         }
         // $listMapper