Method getInstance

/**
 * Returns a reference to the ChangeSet object, only creating it if it doesn't already exist.
 *
 * @param   DatabaseDriver  $db      The current database object
 * @param   string          $folder  The full path to the folder containing the update queries
 *
 * @return  ChangeSet
 *
 * @since   2.5
 */
public static function getInstance($db, $folder = null)
{
    if (!\is_object(static::$instance)) {
        static::$instance = new static($db, $folder);
    }
    return static::$instance;
}