Replace layer option is enabled even when no layer is currently selected

Steps to Repro:
1. Open attached or create a new project
2. Hierarchy > right click
3. 3D object > terrain
4. Select paint texture from paint tab
5. Edit terrain layer > Check the drop-down panel

Actual Result:
1. Replace layer option is enabled even though no layer is currently selected

Expected Result:
1. Replace layer option should be disabled when a layer is selected (Same as remove layer option)

Occurring with:
2018.3.0a6, 2018.3.0a5, 2018.3.0a4

Note:
New Terrain tools and texture layers are introduced from alpha 4

Environment:
Windows and Mac