Ajax Errore because of get_current_screen() call

Woocommerce 3.2.6. In public function get_post_metadata( $value, $object_id, $meta_key )

$screen = get_current_screen(); if ( 'add' === $screen->action ) // [etc...]

determines a “Failed to load resource: the server responded with a status of 500 (Internal Server Error)” in console and when you try to add via select a new custom attribute the wheel spins infinitely and you get “load-scripts.php?c=0&load[]=jquery-core,jquery-migrate,underscore,utils,jquery-ui-core,jquery-ui-widget,jquery-ui-mouse,jquery-ui-sortable,backbone,wp-util,&load[]=wp-backbone,media-models,plupload&ver=4.7.9:4 POST https://avianieditori/wp-admin/admin-ajax.php 500 (Internal Server Error)” in console

If you delete the get_current_screen() all works!