It looks like the Liquibase cannot connect to the database. It is trying to call driver.connect()on the given JDBC URL and driver and that method is returning null. According to the JDBC Javadoc:
I would double-check your JDBC URL and driver attributes on your <database> and <referencedatabase> elements and make sure it is correct and pointing to the right place.The driver should return "null" if it realizes it is the wrong kind of driver to connect to the given URL.