How to reproduce:
1. Open attached project "LevelDesignTraining.zip"
2. Open "001 MasionInWoods" scene
3. In Hierarchy, select "Terrain" GameObject
4. In Terrain Component, select "Paint Texture"
5. Select any Terrain Layer
6. Try to paint the Texture in any other Terrain in the scene

Expected result: Texture is painted
Actual result: Texture is not painted and errors are thrown

Reproducible with - 2019.2.0a13, 2019.3.0a1

Thrown errors:
1) NullReferenceException: Object reference not set to an instance of an object
2) Matrix stack full depth reached

Notes: Could not reproduce with 2019.1, 2018.3 and 2017.4 because of Editor errors