Crashed when using undo (Ctrl + Z) after flattening a terrain highmap.

Steps to reproduce
- Open the attached Unity Project in 2018.1.0b1.
- Select the "Lower (Flatten to 50)" game object"
- Paint in height detail into the terrain using a big brush.
- Use the "Flatten" option in the terrain inspector.
- Spam "Ctrl + Z" to undo the flatten.
Notice issue >> Unity Crashes.

NOTE:
I've been unable to repo this issue.