How to reproduce:
1. Open the user's project
2. Enter Play Mode

Expected result: Editor enters Play Mode
Actual result: Editor crashes

Reproducible on 2018.3.0a4, 2018.3.2f1, 2019.1.0a13
Not reproducible on 2017.4.18f1, 2018.3.0a3

Last 5 lines of the stack trace:
0x0000000141C8D363 (Unity) TerrainDataScriptingInterface::GetSplatPrototypes
0x00000001416D78A5 (Unity) TerrainData_Get_Custom_PropSplatPrototypes
0x000000005A246A33 (Mono JIT Code) (wrapper managed-to-native) UnityEngine.TerrainData:get_splatPrototypes (UnityEngine.TerrainData)
0x000000005A24643B (Mono JIT Code) [E:\Downloads\cases\2018.3.0f2\case_1111018-TerrainTest\Assets\CrashTest.cs:9] CrashTest:Start ()
0x000000005A39EBD8 (Mono JIT Code) (wrapper runtime-invoke) object:runtime_invoke_void__this__ (object,intptr,intptr,intptr)

Fixed in: 2019.2.0a8