How to reproduce:
1. Create a new project
2. Create a Terrain (right-click in the Hierarchy tab -> 3D object -> Terrain)
3. Add one more Inspector tab (right-click on any tab -> Add Tab -> Inspector)
4. Notice that one inspector window has the Terrain Inspector disabled under the message "Inspector tab is a duplicate. Paint functionality disabled"
5. Close the functional inspector

Expected result: Terrain paint functionality is working
Actual result: Terrain paint functionality is disabled

Reproducible: 2018.3.0b7, 2018.3.3f1, 2019.1.0a14, 2019.2.0a1
Not reproducible: 2017.4.18f1, 2018.3.0b6, 2019.1.0a7 (terrain is editable with both inspectors)

Notes:
- Terrain paint functionality is disabled, even when deleting or creating a new terrain