Steps to reproduce:
1. Open the attached "terrain213.zip" project
2. Open the SampleScene located in Assets\\Scenes
3. In the Hierarchy window select Terrain
4. In the Inspector window, under the Terrain component select the Paint Terrain tool
5. Observe the Console window

Expected result: the are no errors displayed
Actual result: there are multiple errors displayed, one of which reads: "Retrieving array element that was out of bounds"

Reproducible with: 2021.2.15f1, 2021.2.19f1, 2021.3.1f1
Not reproducible with: 2019.4.38f1, 2021.2.14f1, 2022.1.0f1, 2022.2.0a11

Notes:
- Reproduction requires any version of the Terrain Tools package to be installed (reproduces using any package version in the affected Editor versions)
- Can also be reproduced by selecting different Terrain tools under the Paint Terrain tool
- If it doesn't reproduce on the first try, open any other scene and then repeat steps from step 2
- Selecting different Terrain tools can also sometimes "break" the Inspector window
- Disabling and then enabling the Terrain component resolves these issues
- First few error messages:
"Retrieving array element that was out of bounds"
"NullReferenceException: Object reference not set to an instance of an object"