TMap add error in singleton

Hi. guys.
I’m newby in unreal.
I knew singleton class setting is good for manager. So I make manager class. And I declare some sub manager class for ineventory. Then I declare TMap variable. It works when I find but when I use add or emplace, editor is killed. I did’nt use UPROPERTY as ue4 document said. I don’t know what I miss. Anyone know it?

here is the code and error message

//Singleton
UCLASS()
class SAVINGGAME_API UGameManager : public UObject
{
 GENERATED_BODY()
 
public :
 UGameManager();
 UInventoryManager* _inventoryManager; 
};

//Inventory Manager
UCLASS()
class SAVINGGAME_API UInventoryManager : public UObject
{
GENERATED_BODY()

public:
    TMap<int, int> _itemMap;
    
public:
    UInventoryManager();
 
public:
    UFUNCTION()
    void AddItem(int in_itemID, int in_count = 1);
 
};

//Add item define
void UInventoryManager::AddItem(int in_item, int in_count)
{    
    int* countPtr = _itemMap.Find(in_item);

    if(nullptr == countPtr)
    {
        _itemMap.Add(in_item, in_count);
    }
    else
    {
        (*countPtr) += in_count;
    }
}

238742-