Steps to reproduce:

1. In a new project, create a cube
2. Set the new cube as static
3. In the Mesh Renderer, in Lightmap Settings section, open the Baked Lightmap foldout

Steps to reproduce:

1. Hierarchy > Right click 3D model > Create a new terrain gameobject
2. Select the terrain gameobject in the hierarchy.
3. In the inspector window, select the baked lightmaps dropdown.
4. Observe the crash everytime you click on baked lightmaps dropdown.

Expected Result:
The editor does not crash

Reproduced in:
2017.3.0b1, 2017.3.0b3, 2017.3.0b4, 2017.3.0b5, 2017.3.0b6, 2017.3.0b7

Did not reproduce in:
5.6.4f1, 2017.1.2f1, 2017.2.0f3, 2017.3.0a7

Regression introduced in:
2017.3.0b1