Exception in `IndexGenerator::generatesIndex` when served as deployed war

[tenzap]

I'm serving css-validator as an unpacked war (in jetty)

The logs report

java.io.FileNotFoundException: /run/schroot/mount/dev/usr/share/css-validator/css-validator/WEB-INF/classes/org/w3c/css/index/../../../../validator.html.de (Permission denied)
        at java.base/java.io.FileOutputStream.open0(Native Method)
        at java.base/java.io.FileOutputStream.open(FileOutputStream.java:293)
        at java.base/java.io.FileOutputStream.<init>(FileOutputStream.java:235)
        at java.base/java.io.FileOutputStream.<init>(FileOutputStream.java:184)
        at org.w3c.css.index.IndexGenerator.generatesIndex(IndexGenerator.java:128)
        at org.w3c.css.servlet.CssValidator$1.run(CssValidator.java:164)

It is actually expected that the dir is not writable.

It looks like IndexGenerator::generatesIndex doesn't check for the files at the right location, and believes it has to create it (in the wrong location).