There's a function call to check. Assuming it's like I expect, everything looks ok.

Ok, when I try to save an example project to a new folder after modifying the custom function, I get the error:

C:\Users\gnn85523\AppData\Local\miniconda3\envs\rascal2\lib\site-packages\ratapi\project.py:963: UserWarning: Could not write custom file path as relative to the project directory, which means that it may not work on other devices. If you would like to share your project, make sure your custom files are in a subfolder of the project save location. "path": try_relative_to(item.path, filepath.parent),