Fix for undefined default

Hi, thanks for your plugin.
I suggest a fix for “Notice: Undefined index: default in /wp-includes/class-wp-list-util.php on line 169”.

$new_fields = array(
    'description' => array(
        'title'   => __( 'Description', 'smdfw' ),
        'type'    => 'textarea',
        'default' => NULL,
    ),
);

You can also add “desc_tip” and “description” (eg. this text is shown in frontend).