This is a common and longstanding crash that has affected many users since at least 4.11.
Source context not available (error in user code?).
User Descriptions
repro steps currently unknown
Fatal error: [File:D:\Build\++UE4+Release-4.17+Compile\Sync\Engine\Source\Runtime\CoreUObject\Private\Templates\Casts.cpp] [Line: 10] Cast of nullptr to Object failed UE4Editor_CoreUObject!CastLogError() [casts.cpp:11] UE4Editor_Kismet!CastChecked<UObject,UObject>() [casts.h:211] UE4Editor_Kismet!FTabPayload_UObject::CastChecked<UObject>() [workflowuobjectdocuments.h:35] UE4Editor_Kismet!FDocumentTabFactoryForObjects<UTimelineTemplate>::IsPayloadSupported() [workflowuobjectdocuments.h:101] UE4Editor_Kismet!FDocumentTracker::FindSupportingFactory() [workflowtabmanager.cpp:659] UE4Editor_Kismet!FDocumentTracker::OpenNewTab() [workflowtabmanager.cpp:601] UE4Editor_Kismet!FDocumentTracker::OpenDocument() [workflowtabmanager.cpp:529] UE4Editor_Kismet!FBlueprintEditor::OpenDocument() [blueprinteditor.cpp:7890] UE4Editor_Kismet!FBlueprintEditor::NavigateTab() [blueprinteditor.cpp:7895] UE4Editor_Kismet!TBaseSPMethodDelegateInstance<0,FBlueprintEditor,0,TTypeWrapper<void> __cdecl() [delegateinstancesimpl.h:327] UE4Editor_Kismet!TBaseSPMethodDelegateInstance<0,FBlueprintEditor,0,void __cdecl() [delegateinstancesimpl.h:434] UE4Editor_GraphEditor!SGraphEditorImpl::OnMouseButtonDown() [sgrapheditorimpl.cpp:72] UE4Editor_Slate!<lambda_7097b900f0e4be30d80fb18f98c51ea0>::operator() [slateapplication.cpp:5104] UE4Editor_Slate!FEventRouter::Route<FReply,FEventRouter::FBubblePolicy,FPointerEvent,<lambda_7097b900f0e4be30d80fb18f98c51ea0> >() [slateapplication.cpp:232] UE4Editor_Slate!FSlateApplication::RoutePointerDownEvent() [slateapplication.cpp:5093] UE4Editor_Slate!FSlateApplication::ProcessMouseButtonDownEvent() [slateapplication.cpp:5050] UE4Editor_Slate!FSlateApplication::OnMouseDown() [slateapplication.cpp:4968] UE4Editor_Core!FWindowsApplication::ProcessDeferredMessage() [windowsapplication.cpp:1726] UE4Editor_Core!FWindowsApplication::DeferMessage() [windowsapplication.cpp:2161] UE4Editor_Core!FWindowsApplication::ProcessMessage() [windowsapplication.cpp:881] UE4Editor_Core!FWindowsApplication::AppWndProc() [windowsapplication.cpp:721] user32!UserCallWinProcCheckWow() user32!DispatchMessageWorker() UE4Editor_Core!FWindowsPlatformMisc::PumpMessages() [windowsplatformmisc.cpp:1009] UE4Editor!FEngineLoop::Tick() [launchengineloop.cpp:3151] UE4Editor!GuardedMain() [launch.cpp:166] UE4Editor!GuardedMainWrapper() [launchwindows.cpp:134] UE4Editor!WinMain() [launchwindows.cpp:210] UE4Editor!__scrt_common_main_seh() [exe_common.inl:253] kernel32!BaseThreadInitThunk() ntdll!RtlUserThreadStart()
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-50836 in the post.