Crash when i add element to TMap in USaveGame class

Hi, i have such problem:

I’ve created class UAHSaveGame : public USaveGame
with public variable TMap Levels;

Creating mechanism is: UGameplayStatics::CreateSaveGameObject(UAHSaveGame::StaticClass());

When i run my game as Standalone game, and call Levels.Add(0,0); from Widget
game crashes

SEGV_NOOP at 0x0

FSetElementId TSet, TDefaultMapHashableKeyFuncs, FDefaultSetAllocator>::Emplace >(TPairInitializer&&, bool*) Address = 0x16a59372e [/Users/Shared/EpicGames/UE_4.18/Engine/Source/Runtime/Core/Public/Containers/SparseArray.h, line 110] [in UE4Editor-MyProject-4716.dylib]

Thanks to ExtraLifeMatt, from forum, i have solution. All i need to do is add UPROPERTY() macro, because TMap was garbage collected.