private @Nullable WorkItemProgressEvent toWorkItemProgressEvent(InternalProgressEvent event, InternalWorkItemDescriptor descriptor) {
        if (event instanceof InternalOperationStartedProgressEvent) {
            return workItemStartedEvent((InternalOperationStartedProgressEvent) event, descriptor);
        } else if (event instanceof InternalOperationFinishedProgressEvent) {
            return workItemFinishedEvent((InternalOperationFinishedProgressEvent) event);