Steps to reproduce:

1. Open attached project.
2. Open scene: a
3. Open scene b and c in additive (drag&drop)
4. Create new scene in Additive (ET -> SceneManager -> Additive + Create New Scene)
5. Save SceneSetup (use this custom window ET -> SceneManager)
6. Restore SceneSetup

Error message shows up: ArgumentException: Invalid SceneManagerSetup: One active scene is required. Number of active scenes: 0

This happens when restoring SceneSetup which did not have active scene in it. It did not saved correctly because active scene was Untitled.
To fix this, whether Untitled scene should be saved to SceneSetup or any other (random or first) scene should be made as active.

Reproduced on 5.5.0b1