RxDataSources testing

I have created tableView with sections by RxDataSources.

ViewController

viewModel.sectionsObservable
        .bindTo(tableView.rx.items(dataSource: viewModel.dataSource))
        .disposed(by: disposeBag)

sectionsObservable emits the event when a new element is added to the array.

I would like to test if a proper element is on proper indexPath.

func testGettingTaskForIndexPath() {
    let element = Element()
    viewModel.addElement(element)
    let fetchedElement = viewModel.getElement(at: IndexPath(row: 0, section: 0))
    XCTAssertEqual(element, fetchedElement)
}

ViewModel

func getElement(at indexPath: IndexPath) -> Element {
    return self.dataSource.sectionModels[indexPath.section].items[indexPath.row]
}

it works in the app, but tests fail. fatal error: Index out of range

It's all because of that asynchronous. How to test this case?