Steps to reproduce:

1. Open the attached "LightmapCrash" project and "SceneFramework" scene
2. Enter playmode
3. Repeat 2nd step until crash occurs (usually around 5th time entering playmode)

Result:

========== OUTPUTTING STACK TRACE ==================

0x0000000140763EF6 (Unity) LightmapSettingsManager::CheckSettingsConsistency
0x00000001407680C1 (Unity) SceneLoadedAfterAwake
0x0000000140992C92 (Unity) CallbackArray3<int const ,AwakeFromLoadQueue & __ptr64,enum SceneLoadingMode>::Invoke
0x00000001409924E4 (Unity) LoadSceneOperation::IntegrateMainThread
0x0000000140995E20 (Unity) PreloadManager::UpdatePreloadingSingleStep
0x00000001409968DA (Unity) PreloadManager::WaitForAllAsyncOperationsToComplete
0x0000000140E418A1 (Unity) EditorSceneManager::RestoreSceneBackups
0x0000000141413370 (Unity) PlayerLoopController::EnterPlayMode
0x000000014141F4C8 (Unity) PlayerLoopController::SetIsPlaying
0x0000000141421D5F (Unity) Application::TickTimer
0x00000001415BA215 (Unity) MainMessageLoop
0x00000001415BC4AC (Unity) WinMain
0x000000014239A3FA (Unity) __scrt_common_main_seh
0x00007FFF3AC01FE4 (KERNEL32) BaseThreadInitThunk
0x00007FFF3D1AEFC1 (ntdll) RtlUserThreadStart

========== END OF STACKTRACE ===========

Regression introduced with - 2018.2.0a2
Reproducible with - 2018.2.0a4, 2018.2.0a2
Not reproducible with - 2018.2.0a1, 2018.1.0b10