我正在与gatling合作，并收到此错误，该错误声称“notstartedrefids”已定义。我是加特林的新手，我相信这个价值已经被正确地宣布了，但是我不明白为什么它没有被承认。
包含对值的引用的代码如下：

case object StudentAssignmentScenario {

  private val notStartedRefIdsSession = "notStartedRefIds"

  private val studentAssignmentRefIdSession = "studentAssignmentRefId"

  private val endpoint: String = environmentAssignmentsUrls + "/v2/studentAssignments"

  private val startAssignment = http("Start StudentAssignment")
    .put(endpoint + "/${studentAssignmentRefId}/start")
    .headers(getHeaderMap)
    .check(status.is(200))

  private val submitAssignment = http("Submit StudentAssignment")
    .put(endpoint + "/${studentAssignmentRefId}/submit")
    .headers(getHeaderMap)
    .check(status.is(200))

  private val startAndSubmitAssignments = exec { session =>
    val refId = session(notStartedRefIdsSession).as[Seq[String]].apply(0)
    session.set(studentAssignmentRefIdSession, refId)
  }.exec(startAssignment).exec(submitAssignment)

  private val startAssignments = exec { session =>
    val refId = session(notStartedRefIdsSession).as[Seq[String]].apply(1)
    session.set(studentAssignmentRefIdSession, refId)
  }.exec(startAssignment)

  def readStudentAssignments = exec(http("Get Not Started StudentAssignments")
    .get(endpoint)
    .headers(getHeaderMap)
    .queryParamMap(Map[String, String]("inclExpired" -> "true",
      "context" -> "basel",
      "sort" -> "dueDate,title",
      "limit" -> "25", "offset" -> "0",
      "status" -> "NOT_STARTED"))
    .check(status.is(200),
      // save refIds
      jsonPath("$.items[?(@.status == 'NOT_STARTED')].refId").findAll.optional.saveAs("notStartedRefIds")))

  private val studentAssignmentWorkflow = exec(startAndSubmitAssignments)
    .exec(startAssignments)

  val studentAssignmentWorkflowSetup: ChainBuilder = exitBlockOnFail(
    group("Student Assignment Workflow") {
      studentAssignmentWorkflow
    })
}