I'm also getting this error. I tried deleting the save folder but no luck. I'm using the Jar. I'm also on Java 16.0.2.0 if that's relevant.
Error Log:
It's probably not going to work with Java 16.0.2.0 - you should use the java runtime that comes bundled with the game. If you want to download your own copy of it, it's jdk1.8.0_45. EDIT: for posterity, the game now works with more recent versions of Java, and definitely should not be run with 1.8.0_45.