Crashing on launch. Please help!

Hello, I have a really important project that started to crash without any reason. What can I do to fix this?

MachineId:38A4F0C34129E08EB8A2A69761C2AA8A
EpicAccountId:e93c7807151b417ca5609d206a704228

Fatal error: [File:D:\Build++UE4+Release-4.12+Compile\Sync\Engine\Source\Runtime\Foliage\Private\FoliageInstanceBase.cpp] [Line: 90]
Instance base cache - integrity verification(3): Counter: 23 Size: 11, InvSize: 9 (L:1 S:0 T:0)

UE4Editor_Core!FDebug::AssertFailed() [d:\build++ue4+release-4.12+compile\sync\engine\source\runtime\core\private\misc\outputdevice.cpp:440]
UE4Editor_Foliage!operator<<() [d:\build++ue4+release-4.12+compile\sync\engine\source\runtime\foliage\private\foliageinstancebase.cpp:93]
UE4Editor_Foliage!AInstancedFoliageActor::Serialize() [d:\build++ue4+release-4.12+compile\sync\engine\source\runtime\foliage\private\instancedfoliage.cpp:2247]
UE4Editor_CoreUObject!FLinkerLoad::Preload() [d:\build++ue4+release-4.12+compile\sync\engine\source\runtime\coreuobject\private\uobject\linkerload.cpp:3293]
UE4Editor_CoreUObject!EndLoad() [d:\build++ue4+release-4.12+compile\sync\engine\source\runtime\coreuobject\private\uobject\uobjectglobals.cpp:1411]
UE4Editor_CoreUObject!LoadPackageInternal() [d:\build++ue4+release-4.12+compile\sync\engine\source\runtime\coreuobject\private\uobject\uobjectglobals.cpp:1171]
UE4Editor_CoreUObject!LoadPackageInternal() [d:\build++ue4+release-4.12+compile\sync\engine\source\runtime\coreuobject\private\uobject\uobjectglobals.cpp:1286]
UE4Editor_UnrealEd!UEditorEngine::Map_Load() [d:\build++ue4+release-4.12+compile\sync\engine\source\editor\unrealed\private\editorserver.cpp:2397]
UE4Editor_UnrealEd!UEditorEngine::HandleMapCommand() [d:\build++ue4+release-4.12+compile\sync\engine\source\editor\unrealed\private\editorserver.cpp:5987]
UE4Editor_UnrealEd!UEditorEngine::Exec() [d:\build++ue4+release-4.12+compile\sync\engine\source\editor\unrealed\private\editorserver.cpp:5477]
UE4Editor_UnrealEd!UUnrealEdEngine::Exec() [d:\build++ue4+release-4.12+compile\sync\engine\source\editor\unrealed\private\unrealedsrv.cpp:744]
UE4Editor_UnrealEd!FEditorFileUtils::LoadMap() [d:\build++ue4+release-4.12+compile\sync\engine\source\editor\unrealed\private\filehelpers.cpp:2135]
UE4Editor_UnrealEd!FEditorFileUtils::LoadDefaultMapAtStartup() [d:\build++ue4+release-4.12+compile\sync\engine\source\editor\unrealed\private\filehelpers.cpp:3349]
UE4Editor_UnrealEd!FUnrealEdMisc::OnInit() [d:\build++ue4+release-4.12+compile\sync\engine\source\editor\unrealed\private\unrealedmisc.cpp:300]
UE4Editor_UnrealEd!EditorInit() [d:\build++ue4+release-4.12+compile\sync\engine\source\editor\unrealed\private\unrealed.cpp:89]
UE4Editor!GuardedMain() [d:\build++ue4+release-4.12+compile\sync\engine\source\runtime\launch\private\launch.cpp:137]
UE4Editor!GuardedMainWrapper() [d:\build++ue4+release-4.12+compile\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:126]
UE4Editor!WinMain() [d:\build++ue4+release-4.12+compile\sync\engine\source\runtime\launch\private\windows\launchwindows.cpp:200]
UE4Editor!__scrt_common_main_seh() [f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:264]
kernel32
ntdll

Hello,

This is currently a known issue: Unreal Engine Issues and Bug Tracker (UE-32774)

The issue has been resolved in 4.13, and you can grab the commit here if you are using source: https://github.com/EpicGames/UnrealEngine/commit/bac67b96d7f1d4323db570c6a0485cdfb0414c23

Have a great day!

Just to clarify, if you move your project to 4.13 it will no longer crash.