Graph is linked to private object(s) in an external package. /Engine/Transient error

Development Programming & Scripting Blueprint
, , , ,
1

Keep getting this error but it looks untraceable. I’ve read that there was a bug in older UE versions, that caused this. Any chance it’s still not fixed? Or is it me who screw up?

UE4 ver. 4.14.3
“Can’t save BattlePreparations.uasset: Graph is linked to private object(s) in an external package.
External Object(s):
/Engine/Transient”

What exactly do I do:
I have a list of all available units stored in my custom game instance. They are held in custom struct.

USTRUCT()
struct FUnitInfo
{
	GENERATED_BODY()

public:
	UPROPERTY(EditAnywhere, BlueprintReadOnly)
		TSubclassOf<AUnit> UnitClass;
};

So there’s a public UPROPERTY in game instance:

UPROPERTY(EditAnywhere, BlueprintReadWrite)
	TArray<FUnitInfo> Units;

And a function that returns this array. I call this function in blueprints and create a horizontalBox children for each array element.
I keep getting another warning tho, when I am trying to get certain element of this array:

BlueprintEditorCompileResults:Warning: Warning  Item_UnitClass  - the native property is not tagged as BlueprintReadWrite or BlueprintReadOnly, the pin will be removed in a future release.
BlueprintEditorCompileResults:Warning: Warning  K2Node_CallArrayFunction_22  has no property tagged as BlueprintReadWrite or BlueprintReadOnly. The node will be removed in a future release.

But as you can see - I have those flags on. What could be the problem?