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