If the File Association property file has an icon specification with relative path name such as:
icon=config/myicon.ico
wix will crash with error that it cannot find icon.
If full path is given:
icon=/home/aherrick/jpackage-lite/windows/config/myicon.ico
everything works fine and that icon is used.

can be fixed by just setting fa.icon to be an absolute path in FileAssociation.fetchFrom:
                        assoc.iconPath = icon.toPath().toAbsolutePath();