To reproduce:

1. Open the project, attached by the tester (sceneCrash.zip)
2. Make sure that scene1 and scene2 are open at the same time
3. Delete Directional Light game object from scene2
4. Delete the scene2 asset from the Project
5. Right click on scene2 in the Hierarchy and press Discard Changes
6. In the dialog, press OK

Expected: discarding changes for the scene that doesn't exist should not crash the Editor. Alternatively, Editor should know when the scene was removed from the project and should close it and remove from the Hierarchy automatically

Reproduced in 5.5.4f1, 5.5.2f1, 5.6.0f3, 5.6.2p2, 2017.1.0f3, 2017.2.0b2

Fixed in 2017.3.0a5