ResearchKit stepResults startTime和endTime

Question

在最新版本的Swift和Xcode中，我有以下代码

  let taskResult = taskViewController.result
    let stepResult = taskResult.stepResult(forStepIdentifier: ORKStroopStepIdentifier)
    if let result = stepResult?.firstResult as? ORKStroopResult
    {
        print(result.startTime)
        print(result.endTime)
    }

程序无法构建，并显示以下错误：

:-1: linker command failed with exit code 1 (use -v to see invocation)

任何帮助将不胜感激