To those who are looking for a way even though time has passed.
Use FClassFinder
static ConstructorHelpers::FClassFinder<UUserWidget> ErrWdgtCntnt(TEXT("/Game/UI/Widgets/Common/WGT_Warning'"));
if (ErrWdgtCntnt.Succeeded())
{
ErrorDialogWidget = ErrWdgtCntnt.Class;
}