Steps to reproduce:
1. Open a new project and create a Terrain
2. Create a couple of Terrain Layers in Terrain Inspector window
3. Delete them in Project Window

Expected result: You are able to remove deleted layers from "Terrain Layers" in the Inspector window.
Actual result: You are unable to remove deleted layers because you cannot select them

Reproduced in: 2018.3.0a8, 2018.3.7f1, 2019.1.0b5, 2019.2.0a6
Not reproduced: 2018.3.0a4 - 2018.3.0a7

Note:
1. As a workaround, you can delete all layers one by one, but that's clearly not a realistic solution.
2. Could not reproduce in versions prior to 2018.3.0a4 because Terrain Layer were not present there