Steps to repro:

1. Open attached project and 'main' scene;
2. Drag 't1' scene to the Hierarchy;
3. Open Lighting window (Window>Lighting>Settings);
4. Click 'Generate Lighting' button;
5. Wait until baking is finished.

Actual result:

Editor crashes (see attached video). Last calls from the stacktrace:

0x0000000141F251F7 (Unity) AssetDatabase::CheckCanAddObjectToAsset
0x0000000141F26889 (Unity) AssetDatabase::CreateSerializedAsset
0x00000001411582AD (Unity) WriteLightingDataAssetJob::Integrate
0x0000000140DE69FB (Unity) JobManager::IntegrateCompletedJobs
0x0000000141142A05 (Unity) GISceneManager::Update
0x000000014113CD95 (Unity) `GISceneManager::GISceneManager'::`2'::tickGIInEditorRegistrator::Forward

Regression introduced in Unity 2018.2.0a3.

Workaround is to bake lighting in auto-mode.

Expected result:
Editor should not crash after generating lighting for multiple scenes.

Notes:
- Reproducible in 2018.2.0a3;
- Not reproducible in 2018.2.0a2, 2018.1.0b10, 2017.3.0p2;
- Reproducible on Windows 10 and OSX 10.13.3;
- Reproducible when Enlighten is set as baking backend for both scenes; when PLM is set, the Editor spams errors (known bug);
- Reproducible only when manually baking scenes; not reproducible in auto-mode;
- The bug completely prevents manual baking of additively loaded scenes (since PLM batch baking has known issues currently).