It's occurred to me that I recompiled the jar with Java 11 at some point so it likely won't run on older versions of Java. My apologies for any inconvenience, but until this is resolved if you encounter any problems running the game please try upgrading to either Java 11 or a newer version of Java. I will recompile the jar file for Java 8 when I have time, but due to changes between Java 8 and Java 11 it may take some time.