Reproduction steps:
1. Open attached project ("m2 MissingTree")
2. Delete the "Tree" prefab in the Project window
3. Enter and Quit Play mode

Expected: Editor does not Crash after quitting Play mode
Actual: Editor Crashes after quitting Play mode

Reproduced in: 2017.2.4f1, 2017.4.8f1, 2018.1.9f2, 2018.2.11f1, 2018.3.0b4, 2019.1.0a3

Notes:
Issue does not occur when Grass Detail Mesh or Grass Texture prefab is deleted
Editor crashes upon entering playmode in Unity 2018.3 and 2019.1

Stack trace:
0x000000014101601D (Unity) gi::ExtractUniqueMaterials
0x0000000141025D5A (Unity) gi::UpdateMaterials
0x0000000141025489 (Unity) GISceneManager::Update
0x000000014101F665 (Unity) `GISceneManager::GISceneManager'::`2'::tickGIInEditorRegistrator::Forward
0x000000014063BAF9 (Unity) CallbackArray::Invoke
0x00000001412D4A1F (Unity) Application::TickTimer
0x0000000141441312 (Unity) MainMessageLoop
0x00000001414435AE (Unity) WinMain
0x00000001423B57DA (Unity) __scrt_common_main_seh
0x00007FFD77113034 (KERNEL32) BaseThreadInitThunk
0x00007FFD77261431 (ntdll) RtlUserThreadStart

Update:
Fixed in 2019.1.0a6