The Wordpress 4.9.1. has just been released. I have tried to enable the customize tag e.g. script in my post, and it throws the following warning:

Warning: in_array() expects parameter 2 to be array, null given in wp-includes/kses.php on line 1416

Screenshot:

The steps to reproduce:

1. add define('CUSTOM_TAGS', true); in wp_config.php under your wordpress root directory.
2. add the following to functions.php template

function add_scriptfilter( $string ) {
  global $allowedtags;
  $allowedtags['script'] = array( 'src' => array () );
  return $string;
}

add_filter( 'pre_kses', 'add_scriptfilter' );

and then the warning will be thrown on all pages/posts.

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