I’ve never used that tool, but I can interpret the java exception stack trace you showed for you.
Reading from the bottom to the top, it got stuck in the component that does the actual conversion from Minecraft world to HL2 VMF file.
There, it is still in “getMinecraftMap”, which probably means “let’s read the Minecraft data or info about the world so we can process it”.
There, it counts files, probably with the .mca ending (the actual “area” files with the block data in them). For that, it’ll probably use a system call to enumerate files with a search mask, e.g. your folder*.mca. And then the person who programmed that probably didn’t write a handling for when no files were found at all and the call returned null instead of a list of files in that case.
So I guess the culprit is that you haven’t configured the source directory with the minecraft world right, or that you did but the tool relies on an outdated directory structure. What you can do is point at yourworld\ (without region\ at the end), or yourworld\region\ for the overworld dimension if you already tried that, or maybe it requires you to type out *.mca at the end, etc.
It’ll probably give an example somewhere in the accompanying documentation. 
