```
https://xref.landonf.org/source/xref/haiku/src/kits/package/hpkg/PackageReaderImpl.cpp#193
```
I already seem to have found the cause (and fixed it in my own writer).
Haiku's package reader calls the handler callback whenever it meets `B_HPKG_ATTRIBUTE_ID_FILE_ATTRIBUTE`.
Because my `B_HPKG_ATTRIBUTE_ID_FILE_ATTRIBUTE` is listed before `B_HPKG_ATTRIBUTE_ID_SYMLINK_PATH`, `B_HPKG_ATTRIBUTE_ID_SYMLINK_PATH` was still null at the time the symlink was being created.
Therefore, Haiku crashed (while trying to read a null pointer).

But well according to the docs on GitHub this order of attributes is perfectly legal.