This is a somewhat common crash on Mac that has affected users since 4.16. Users have not provided any descriptions of their actions when the crash occurred.
Callstack from Log
[2017.09.08-05.17.43:478][756]Cmd: QUIT_EDITOR [2017.09.08-05.17.43:572][756]LogExit: Preparing to exit. [2017.09.08-05.17.43:629][756]LogMac: === Critical error: === SIGSEGV: invalid attempt to access memory at address 0x0 [2017.09.08-05.17.43:629][756]LogMac: scalable_msize() Address = 0xb58ad6b (filename not found) [in UE4Editor-Core.dylib] FMallocTBB::Realloc(void*, unsigned long, unsigned int) Address = 0xb6b44bb (filename not found) [in UE4Editor-Core.dylib] FMemory::Realloc(void*, unsigned long, unsigned int) Address = 0xb6ba741 (filename not found) [in UE4Editor-Core.dylib] TArray<FDeferredMacEvent, FDefaultAllocator>::ResizeGrow(int) Address = 0xb7ce573 (filename not found) [in UE4Editor-Core.dylib] FMacApplication::DeferEvent(NSObject*) Address = 0xb7e97cb (filename not found) [in UE4Editor-Core.dylib] invocation function for block in FMacApplication::FMacApplication() Address = 0xb7ee01a (filename not found) [in UE4Editor-Core.dylib] _NSSendEventToObservers() Address = 0xa99097fa (filename not found) [in AppKit] -[NSApplication(NSEvent) sendEvent:] Address = 0xa9f0223f (filename not found) [in AppKit] -[NSApplication run] Address = 0xa977d427 (filename not found) [in AppKit] tchar_main(int, wchar_t**) Address = 0xb5230f3 (filename not found) [in UE4Editor] main() Address = 0xb522979 (filename not found) [in UE4Editor] start() Address = 0xc1408235 (filename not found) [in libdyld.dylib]
repro steps currently unknown
SEGV_NOOP at 0x0 UE4Editor-Core.dylib!<Unknown> UE4Editor-Core.dylib!FMallocTBB::Realloc() UE4Editor-Core.dylib!FMemory::Realloc() UE4Editor-Core.dylib!TArray<FDeferredMacEvent, FDefaultAllocator>::ResizeGrow() UE4Editor-Core.dylib!FMacApplication::DeferEvent() UE4Editor-Core.dylib!<Unknown> AppKit!<Unknown> AppKit!-[NSApplication() AppKit!<Unknown> UE4Editor!tchar_main() UE4Editor!<Unknown> libdyld.dylib!<Unknown>
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-49784 in the post.