First repro in 2020.1.0a3 (a37e4d4d532f)
does not repro in 2020.1.0a2 (d3fe9dc000c3)

Repro steps:
1. Create a new 3D project. Do not disable auto-generate lighting.
2. Create a terrain.
3. Click the terrain.
4. In the inspector, go to terrain settings.
Expected result: the terrain settings displays without error
Actual result: once the lightmap starts baking, the terrain settings will collapse into a bugged state and an error will be printed to the console. The terrain inspector will be unusable until the lightmap baking is stopped.