Thanks! I just looked up "sandbox game" on Wikipedia to refresh my memory and it gave me the idea of maybe adding challenges to make it one of those "solve this problem anyway you can" deals. Maybe a puzzle mode? I'll add it to my idea file.

Someone else mentioned that it crashes on mac but didn't provide the output yet so thanks for that. I don't have a mac but I'll have a look to see if I can fix it.

I emailed you the error log I see when trying to run the jar on macOS 10.14. It contains more errors than the log that trungdq88 posted does.

Oh, whoops, I see that the Download page says JRE 11 or newer must be installed. My JRE version was 8 (Java 1.8).

After switching to OpenJDK 14, I get the same errors as trungdq88.