If you can provide reproduction steps, please log a bug report on the UE4 Answerhub.
This is a frequent crash occurring in the 4.15 Preview. No other information is currently known.
Regression?
Yes - there is no evidence of this occurring in 4.14
SEGV_MAPERR at 0x3 UE4Editor-Renderer.dylib!FRWBuffer::Initialize() UE4Editor-Renderer.dylib!FMinimalDummyForwardLightingResources::InitRHI() UE4Editor-MetalRHI.dylib!FMetalDynamicRHI::FMetalDynamicRHI() UE4Editor-MetalRHI.dylib!FMetalDynamicRHIModule::CreateRHI() UE4Editor-RHI.dylib!PlatformCreateDynamicRHI() UE4Editor-RHI.dylib!RHIInit() UE4Editor!FEngineLoop::PreInit() UE4Editor!GuardedMain() UE4Editor!<Unknown> UE4Editor-Core.dylib!<Unknown> Foundation!Unknown() libsystem_pthread.dylib!<Unknown> libsystem_pthread.dylib!<Unknown> libsystem_pthread.dylib!<Unknown>
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-40781 in the post.