To see a NullpointException from the very first start is rather disappointing, so thumbs up for your patience!
To make things reproducible, what OS + verion and JVM + version do you use?
Did you have more luck with other JanusGraph versions?
I just checked with Ubuntu MATE 20.04 with OpenJDK Runtime Environment (build 1.8.0_312-8u312-b07-0ubuntu1~20.04-b07) and did not experience your issue.
Best wishes, Marc