Steps to reproduce:
1. Open user's attached project
2. Select "Transparent" Object, expand it's Material and drag "red" from the assets on "Transmissive Color"
3. Start generating lighting
4. Wait a few seconds and then drag "smallrattle" from the assets on "Transmissive Color"

--Unity stops responding and crashes without the Bug Reporter

Reproduced with: 5.6.5f1, 2017.1.2p4, 2017.2.1p1, 2017.3.0f3, 2018.1.0b1

Note: When starting the bake with "smallrattle" and then changing to "red" it did not crash, however after a few more swaps it eventually did

Fixed in 2018.2.0a3