ios – 已存在标识为backgroundSession的后台URLSession

我正在使用NSURLSession和NSURLSessionDelegate从服务器获取xml数据.取决于我与服务器连接的一些条件.如果我正在连接服务器一切正常,没有任何错误,但如果我没有连接(取决于条件)服务器和移动到另一个视图控制器(通过使用故事板？.instantiateViewControllerWithIdentifier(id))我得到以下 IOS错误：

‘标识符backgroundSession的后台URLSession已经存在！’

这是我的代码：

class MainClass: UITableViewController,NSURLSessionDelegate {     

   var task_service = NSURLSessionDataTask?()

   override func viewDidLoad() {
      super.viewDidLoad()

      if(condition) {
        getXMLFromServer()
      }

   }

   func getXMLFromServer(){

     task_service = getURLSession().dataTaskWithRequest() {

        (data,response,error) -> Void in

        dispatch_async(dispatch_get_main_queue(),{

         // Fetching data from server 

         // In the end
         self.session.invalidateAndCancel()
       }
    }

  }

 func getURLSession() -> NSURLSession {

    let configuration =      NSURLSessionConfiguration.defaultSessionConfiguration()

    configuration.timeoutIntervalForRequest = 30.0

    session = NSURLSession(configuration: configuration,delegate: self,delegateQueue: NSOperationQueue.mainQueue())

    return session
  }

 func URLSession(session: NSURLSession,task: NSURLSessionTask,didReceiveChallenge challenge: NSURLAuthenticationChallenge,completionHandler: (NSURLSessionAuthChallengedisposition,NSURLCredential?) -> Void) {

    completionHandler(NSURLSessionAuthChallengedisposition.UseCredential,NSURLCredential(forTrust: challenge.protectionSpace.serverTrust!)) // Bypassing SSL error
 }   
}

编辑：找到错误的原因.

由于在Called View Controller中创建了NSURLSession而发生错误.Called VC包含从服务器下载PDF的代码.但我不知道如何解决这个问题.下面是Called VC的代码

class MainFormsController: UIViewController,UIPickerViewDelegate,UITextFieldDelegate,NSURLSessionDownloadDelegate,UIDocumentInteractionControllerDelegate,MFMailComposeViewControllerDelegate{

 var download_task = NSURLSessionDownloadTask?()
 var backgroundSession = NSURLSession()

 override func viewDidLoad() {
    super.viewDidLoad()

     createNSURLSession()
 }               

 /** Error occurred while creating this NSURLSession **/

    func createNSURLSession() { 

        let backgroundSessionConfiguration =  NSURLSessionConfiguration.backgroundSessionConfigurationWithIdentifier("backgroundSession")

       backgroundSession = NSURLSession(configuration:   backgroundSessionConfiguration,delegateQueue:   NSOperationQueue.mainQueue())
    }

  func downloadPDF() {

     //Download PDF
     download_task = backgroundSession.downloadTaskWithURL(url)
     download_task?.resume()
  }

}

deinit {
        self.backgroundSession.finishTasksAndInvalidate();
    }