To reproduce:
1. Open attached project
2. Open "SceneA" scene
3. Play the scene
4. The script will first additively load SceneB, after 3 seconds it will unload SceneA and after 6 seconds it will try to load SceneA. After this it will crash.

Actual Result: pop-up window with message "GetManagerFromContext: pointer to object of manager 'LightmapSettings' is NULL (table index 25)" and crash

In 5.3.0b6 only warnings appear: "Unloading the last scene Assets/SceneA.unity(index: 0), is not supported. Please use SceneManager.LoadScene()/EditorSceneManager.OpenScene() to switch to another scene."

Not reproducible with: 5.3.0b6
Reproducible with: 5.3.0f1, 5.3.0f2