Many packages report this (benign?) warning:

--8<---------------cut here---------------start------------->8---
starting phase `patch-dot-desktop-files'
adjusting 2 '.desktop' files in "/gnu/store/j8q725jv5my3gcqi1kgb2g99839xcbap-racket-7.3/share/applications"
warning: '.desktop' file refers to '', which cannot be found
...
--8<---------------cut here---------------end--------------->8---

Racket is one example.

The .desktop file is fine though, there is no empty string nor broken references.

-- 
Pierre Neidhardt
https://ambrevar.xyz/