Steps to reproduce:
- Open attached project and scene named "library" which can be found under Assets/Scenes
- On the top menu select Window > Lighting
- Press "Build" which is placed on newly opened "Lighting" window's bottom
- Notice that built lighting folder named "library" (name depends on the scene's name) was created and placed in the Scenes folder
- Close Unity and re-open the same project
- Notice error which complains about newly created folder named "library" (see attached image)

Expected behavior:
There should be no error no matter what is the name of the built lighting folder

Actual behavior:
Unity gives an error about multiple folders and does not let open the project

Note:
The error does not occur if the "library" folder is empty

Workaround:
Change scene's name to anything but "library" and then do the build of lighting

Reproduced with:
5.4.0b19, 5.4.0b1, 5.3.5f1, 5.3.4f1