Steps to reproduce:
1. Open the attached project "Case_1159201"
2. Navigate to Project -> Assets and open "TestBug"
3. In the hierarchy window select "Left" or "Right" GameObject
3. Enter Play mode
-- Notice the console "NullReferenceException: Object reference not set to an instance of an object" error

Expected results: No errors are displayed when creating a Terrain with a script while Terrain Tool package is installed
Actual Results: NullReferenceException is thrown when creating a Terrain with a script while GameObject with a script is selected in the Hierarchy

Reproducible with: 2019.1.0f2(preview - 1.1.2), 2019.1.5f1(preview - 1.1.2), 2019.2.0b5(preview - 1.1.2), 2019.3.0a4(preview - 1.1.4)

Notes: Terrain Tool package appeared in 2019.1. Versions lower than 2019.1.0f2 crash and throw package errors. Versions lower than 2019.2.0a8 throw package errors.