Repro Steps:
1. Install Terrain Tools package 3.0.2-preview.2
2. Create a Terrain Object in the scene
3. Select the Paint Terrain Button
4. Change to the Set Height Tool
5. Press the Flatten Terrain button

Actual Results: An exception occurs, warnings will spam the console, and the action will not be completed.
Expected Results: No errors would occur and the action would successfully complete

Notes:
- It's important that SetHeight is the first action taken on the Terrain object to reproduce the issue. Issue will not reproduce if you paint on the terrain first.
- Warnings will continue to spam the console log until you restart the editor. You can even delete the Terrain object but the editor will still spam warnings.