Doesn't work on Linux due to filesystem structure

[TheKodeToad]

If the csv file is visited first by the lambda, a nullpointerexception is thrown.
At least, this seems to be the problem.

I added debug printing.

@@@@@@@@@@@@@@@@@==@@@@@@@@@@@@@@@@@
@@@@@@@@@@@@@==-------=@@@@@@@@@@@@@
@@@@@@@@@@=-----@@@@@@=---=@@@@@@@@@
@@@@@@@@= -=@-  @=@@@@@@@@- @@@@@@@@         Lunar Remapper
@@@@@@@@- @@    =@@@@@@@=@= =@@@@@@@         Version: 1.0-SNAPSHOT
@@@@@@@@- @@     -=@=@@@=@= =@@@@@@@         Developed by Decencies
@@@@@@@@- @=@-         -@@= =@@@@@@@
@@@@@@@@- -               - -@@@@@@@
@@@@@@@@@@===---     -=-===@@@@@@@@@
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
./mappings/1.8.9/fields.csv
Exception in thread "main" java.lang.NullPointerException
	at club.decencies.remapper.lunar.Main.lambda$main$0(Main.java:58)
	at club.decencies.remapper.lunar.util.FileUtil.walk(FileUtil.java:24)
	at club.decencies.remapper.lunar.Main.main(Main.java:49)

As you can see, the folder layout is all correct.

Personally, I think it's better only to list the directory layout, and create file objects for each of the expected files, that way the order can be guaranteed.

File mappingsDir = new File("mappings");
for (File mappingDir : mappingsDir.listFiles()) {
  File proguardMappings = new File(mappingDir, mappingDir.getName() + ".txt");
  File specialSourceMappings = new File(mappingDir, mappingDir.getName() + ".srg");
  // etc...
}

I also used find in the mappings folder:

.
./1.8.9
./1.8.9/fields.csv
./1.8.9/params.csv
./1.8.9/1.8.9.srg
./1.8.9/methods.csv

The order is not what the program expects, and causes an error. My guess is that Windows does some fancy lexicographical (aka alphabetical but that sounds far more technical) sorting in the filesystem API, which Linux doesn't do, where I believe it's sorted by its actual order in the filesystem.