[ERROR] Assertion failed: !IsUnreachable()

Assertion failed: !IsUnreachable()
[File:D:\Projects\UESources\Engine\Source\Runtime\CoreUObject\Private\UObject\ScriptCore.cpp]
[Line: 991]
InworldMarkersComponent_BP_C
/Engine/Transient.GameEngine_0:BP_ArenaGameInstance_C_0.None
Function:
‘/Script/sZoneDriver.InworldMarkerOtherPlayer:Update’

I use just this code:

mark.Mark->Update(mark.Actor);

where mark is USTRUCT from TArray, and Mark is UUserWidget child class instance.
I added the if (!mark.Mark->IsUnreachable()) into code to check object before using. But in what situations this must be used explicitly? IsValidLowLevel() insufficiently?

#Thanks!