Steps to reproduce:
1. Open the attached project (UnityChangeSceneTerrain.zip)
2. Build and run
3. Core scene will load
4. 5 seconds later a scene with a basic terrain (SceneA) will load
5. 5 seconds later a scene with a new terrain and trees (SceneB) will load
6. It will crash on trying to unload (SceneA) (SceneManager.UnloadSceneAsync("SceneA") -> Assets/LevelManager.cs:63 )

Expected result: Scene should be unloaded
Actual result: Unloading previous scene results in a crash

Note:
If you remove the trees from the terrain on SceneB it will work fine
UnloadSceneAsync was added in Unity 5.5

Reproduced with: 5.5.0a1, 5.5.0b1, 5.5.0f3, 5.5.0p1, 5.6.0a6