How to reproduce:
1. Open Unity
2. Load tester-submitted project(TerrainCrashTest.zip)
3. Open '1.unity' scene
4. Select any 'Terrain' game object.
5. Select 'Paint texture' tab
6. Alter-Select terrains (crash can easily be achieved using arrow keys)

Expected result: Unity successfully selects different terrain pieces
Actual result: Unity crashes and opens bug report window

Notes:
* Having 'Paint Texture' tab open when creating, deleting, duplicating, resizing terrains, exiting 'Play mode' and switching between 'Scene view' and 'Game view' also reproduces this issue but with a different stack trace.

Reproducible with - 2017.3.0f3, 2018.1.0a6, 2018.1.0b1
Not reproducible with - 2017.3.0p1

Fixed and verified in 2017.3.0p1, 2018.1.0b3