This is a common crash affecting Mac users in 4.16, and going back at least to 4.11. This seems to be a continuation of [Link Removed], which was closed as cannot repro.
User Descriptions
From Log
'/Game/StarterContent/Maps/AINavigation_geo.AINavigation_geo:PersistentLevel.AINavigation_geo' LogMetal:Display: Shrinking RingBuffer from 1643264 to 1232640 as max. usage is 44288 at frame 71118] LogMac:Error: appError called: Assertion failed: Assertion failed: Pair != nullptr [File:Runtime/Core/Public/Containers/Map.h] [Line: 457] LogCrashTracker: ]LogMac: === Critical error: === SIGSEGV: invalid attempt to access memory at address 0x3 LogMac: FGenericPlatformMisc::RaiseException(unsigned int) Address = 0x1ff600b (filename not found) [in UE4Editor-Core.dylib] FOutputDeviceMacError::Serialize(wchar_t const*, ELogVerbosity::Type, FName const&) Address = 0x21f1a2f (filename not found) [in UE4Editor-Core.dylib] FOutputDevice::Logf(wchar_t const*, ...) Address = 0x22d26cd (filename not found) [in UE4Editor-Core.dylib] FDebug::AssertFailed(char const*, char const*, int, wchar_t const*, ...) Address = 0x223893e (filename not found) [in UE4Editor-Core.dylib] SWidget::FindChildGeometry(FGeometry const&, TSharedRef<SWidget, (ESPMode)0>) const Address = 0x4579197 (filename not found) [in UE4Editor-SlateCore.dylib] SScrollBar::OnMouseButtonDown(FGeometry const&, FPointerEvent const&) Address = 0x3e465e4 (filename not found) [in UE4Editor-Slate.dylib] FSlateApplication::ProcessMouseButtonDownEvent(TSharedPtr<FGenericWindow, (ESPMode)0> const&, FPointerEvent&) Address = 0x3aa9b02 (filename not found) [in UE4Editor-Slate.dylib] FSlateApplication::OnMouseDown(TSharedPtr<FGenericWindow, (ESPMode)0> const&, EMouseButtons::Type, FVector2D) Address = 0x3b066d6 (filename not found) [in UE4Editor-Slate.dylib] FMacApplication::ProcessMouseDownEvent(FDeferredMacEvent const&, TSharedPtr<FMacWindow, (ESPMode)0>) Address = 0x21c1b19 (filename not found) [in UE4Editor-Core.dylib] FMacApplication::ProcessEvent(FDeferredMacEvent const&) Address = 0x21b804e (filename not found) [in UE4Editor-Core.dylib] ...
repro steps currently unknown
UE4Editor-SlateCore.dylib!SWidget::FindChildGeometry(FGeometry const&, TSharedRef<SWidget,() UE4Editor-Slate.dylib!SScrollBar::OnMouseButtonDown() UE4Editor-Slate.dylib!FSlateApplication::ProcessMouseButtonDownEvent(TSharedPtr<FGenericWindow,() UE4Editor-Slate.dylib!FSlateApplication::OnMouseDown(TSharedPtr<FGenericWindow,() UE4Editor-Core.dylib!FMacApplication::ProcessMouseDownEvent(FDeferredMacEvent const&, TSharedPtr<FMacWindow,() UE4Editor-Core.dylib!FMacApplication::ProcessEvent() UE4Editor-Core.dylib!FMacApplication::ProcessDeferredEvents() UE4Editor-Slate.dylib!FSlateApplication::TickPlatform() UE4Editor-Slate.dylib!FSlateApplication::Tick() UE4Editor!FEngineLoop::Tick() 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-47556 in the post.