When adding a sub-level that contains a Landscape and Landscape Splines to an existing Level, it will cause a crash when building the lighting.
This issue was reported in 4.18.3 (CL 3832480) and tested in 4.19.2 (CL 4033788), 4.20 Preview 3 (CL 4135216), and Main 4.21 (CL 4151283)
Or
Results: The Editor crashes
Expected: Lighting builds and the Editor doesn't crash
[2018.06.21-11.37.48:623][327]LogWindows: Error: [Callstack] 0x0000000028E33C58 KERNELBASE.dll!UnknownFunction [] [2018.06.21-11.37.48:623][327]LogWindows: Error: [Callstack] 0x000000000A597884 UE4Editor-ApplicationCore.dll!FWindowsErrorOutputDevice::Serialize() [d:\build\++ue4+release-4.19+compile\sync\engine\source\runtime\applicationcore\private\windows\windowserroroutputdevice.cpp:65] [2018.06.21-11.37.48:623][327]LogWindows: Error: [Callstack] 0x00000000EDDA6D9B UE4Editor-Core.dll!FOutputDevice::LogfImpl() [d:\build\++ue4+release-4.19+compile\sync\engine\source\runtime\core\private\misc\outputdevice.cpp:70] [2018.06.21-11.37.48:623][327]LogWindows: Error: [Callstack] 0x00000000EDCE1949 UE4Editor-Core.dll!FDebug::AssertFailed() [d:\build\++ue4+release-4.19+compile\sync\engine\source\runtime\core\private\misc\assertionmacros.cpp:419] [2018.06.21-11.37.48:623][327]LogWindows: Error: [Callstack] 0x00000000E8C74413 UE4Editor-Landscape.dll!ULandscapeSplinesComponent::CheckForErrors() [d:\build\++ue4+release-4.19+compile\sync\engine\source\runtime\landscape\private\landscapesplines.cpp:618] [2018.06.21-11.37.48:623][327]LogWindows: Error: [Callstack] 0x00000000EAA8EDF8 UE4Editor-Engine.dll!AActor::CheckForErrors() [d:\build\++ue4+release-4.19+compile\sync\engine\source\runtime\engine\private\actoreditor.cpp:822] [2018.06.21-11.37.48:623][327]LogWindows: Error: [Callstack] 0x00000000E963AACC UE4Editor-UnrealEd.dll!UEditorEngine::Map_Check() [d:\build\++ue4+release-4.19+compile\sync\engine\source\editor\unrealed\private\editorserver.cpp:3758] [2018.06.21-11.37.48:624][327]LogWindows: Error: [Callstack] 0x00000000E962F0C1 UE4Editor-UnrealEd.dll!UEditorEngine::HandleMapCommand() [d:\build\++ue4+release-4.19+compile\sync\engine\source\editor\unrealed\private\editorserver.cpp:5959] [2018.06.21-11.37.48:624][327]LogWindows: Error: [Callstack] 0x00000000E9612FE2 UE4Editor-UnrealEd.dll!UEditorEngine::Exec() [d:\build\++ue4+release-4.19+compile\sync\engine\source\editor\unrealed\private\editorserver.cpp:5443] [2018.06.21-11.37.48:624][327]LogWindows: Error: [Callstack] 0x00000000E9E90489 UE4Editor-UnrealEd.dll!UUnrealEdEngine::Exec() [d:\build\++ue4+release-4.19+compile\sync\engine\source\editor\unrealed\private\unrealedsrv.cpp:696] [2018.06.21-11.37.48:624][327]LogWindows: Error: [Callstack] 0x00000000EBC79D14 UE4Editor-Engine.dll!UEngine::TickDeferredCommands() [d:\build\++ue4+release-4.19+compile\sync\engine\source\runtime\engine\private\unrealengine.cpp:1543] [2018.06.21-11.37.48:624][327]LogWindows: Error: [Callstack] 0x00000000A7F28522 UE4Editor.exe!FEngineLoop::Tick() [d:\build\++ue4+release-4.19+compile\sync\engine\source\runtime\launch\private\launchengineloop.cpp:3506] [2018.06.21-11.37.48:624][327]LogWindows: Error: [Callstack] 0x00000000A7F36BD0 UE4Editor.exe!GuardedMain() [d:\build\++ue4+release-4.19+compile\sync\engine\source\runtime\launch\private\launch.cpp:166] [2018.06.21-11.37.48:624][327]LogWindows: Error: [Callstack] 0x00000000A7F36C4A UE4Editor.exe!GuardedMainWrapper() [d:\build\++ue4+release-4.19+compile\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:144] [2018.06.21-11.37.48:624][327]LogWindows: Error: [Callstack] 0x00000000A7F44177 UE4Editor.exe!WinMain() [d:\build\++ue4+release-4.19+compile\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:223] [2018.06.21-11.37.48:624][327]LogWindows: Error: [Callstack] 0x00000000A7F45B87 UE4Editor.exe!__scrt_common_main_seh() [f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:253] [2018.06.21-11.37.48:624][327]LogWindows: Error: [Callstack] 0x00000000297E8364 KERNEL32.DLL!UnknownFunction [] [2018.06.21-11.37.48:624][327]LogWindows: Error: [Callstack] 0x000000002BCFE821 ntdll.dll!UnknownFunction [] [2018.06.21-11.37.48:624][327]LogWindows: Error: [Callstack] 0x000000002BCFE821 ntdll.dll!UnknownFunction []
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-60909 in the post.