The brush is invisible while editing heightmap, textures, details, and trees the terrain using Prefab editor

Step to Repro:
1. Create a new or open attached project
2. Hierarchy > right click
3. 3D Object > terrain
4. drag and drop terrain in the project window to create a prefab
5. Select the prefab and click "Open Prefab" button
6. Select raise or lower terrain tab
7. Paint heightmap

Actual Result:
1. The brush is invisible while editing the terrain using Prefab editor

Expected Result:
1. Brush should be visible while editing terrain using Prefab editor

Environment:
Windows and Mac

Occurring with:
Version 2018.3.0a2 (e9113db2e9bf)
Branch: prefabs/improved