Sorry when I say "Not Java" I'm not being very helpful. It has of course been compiled with the Java compiler and is running with the Java RE. It's just crap code - concatenating Strings to make file paths is bad Java. Concatenating Strings to make file paths is a very common thing for bad Java programmers to do.
My suggestion - to create that path starting with a directory called "null" in the application's working directory - would probably get you past that error message. If they've created a file path that way, they may well have included more non-portable code that will be as hard or harder for you to kludge your way around. It's not your fault it doesn't work - it's badly written. What you really need to do is to complain to the author.
I see mention of a 'MAC[sic] version" here:
Addon Client MAC v1.09
- is that the file you downloaded?