Prerequisites:
The issue reproduces on DELL XPS laptop having GeForce 640m and integrated Intel HD Graphics 4000 GPUs. For repro it's needed to disable the primary GPU device. Could be done in the Device Manager.

Steps to repro:
1. Open attached project and 't1' scene;
2. In the Lighting settings window select 'Progressive GPU (Preview)' in the Lightmapper dropdown;
3. In the Lighting window press 'Generate Lighting' button.

Expected result:
Message that the current OpenCL device is not supported is shown. Lightmapper falls back to CPU PLM.

Actual result:
Editor crashes (see attached video).

Stacktrace:
0x00007FF7C7B306C2 (Unity) InitOpenCLAndRadeonRaysForGPULightmapper
0x00007FF7C7B22CD1 (Unity) RadeonRaysContextManager::RadeonRaysContextManager
0x00007FF7C7B30865 (Unity) RadeonRaysContextManager::InitializeClass
0x00007FF7C7AA25F6 (Unity) ProgressiveSceneManager::PostUpdate
0x00007FF7C79C391D (Unity) gi::PostUpdate
0x00007FF7C79D5A08 (Unity) GISceneManager::Update
0x00007FF7C79CEF85 (Unity) `GISceneManager::GISceneManager'::`2'::tickGIInEditorRegistrator::Forward
0x00007FF7C4D8F749 (Unity) CallbackArray::Invoke
0x00007FF7C5AB558F (Unity) ProfilerCallbackInvoke<CallbackArray,281736>::Invoke
0x00007FF7C5ABFE20 (Unity) Application::TickTimer
0x00007FF7C5D7E843 (Unity) MainMessageLoop
0x00007FF7C5D881E7 (Unity) WinMain
0x00007FF7C868D40E (Unity) __scrt_common_main_seh
0x00007FFF9A662D92 (KERNEL32) BaseThreadInitThunk
0x00007FFF9C909F64 (ntdll) RtlUserThreadStart

Notes:
- Reproducible in 2019.1.0a13, 2018.3.1f1;
- Reproducible on NVIDIA 640m with NVIDIA 352.63 GameReady driver.