Attempt to present <UIAlertController: > on <ViewController:> whose view is not in the window hierarchy

I am working on a project using ResearchKit, so I send the user into the task I create:

let taskViewController = ORKTaskViewController(task: SurveyTask, taskRunUUID: nil)
taskViewController.delegate = self
presentViewController(taskViewController, animated: true, completion: nil)

When the user is finished with the survey, he goes into:

func taskViewController(taskViewController: ORKTaskViewController, didFinishWithReason reason: ORKTaskViewControllerFinishReason, error: NSError?) {
    switch reason {
        case .Completed:

... }

Here I encounter a problem when I try to show an alert before

taskViewController.dismissViewControllerAnimated(true, completion: nil)

I get the below error:

Attempt to present UIAlertController: ... on ViewController: ... whose view is not in the window hierarchy

Any idea of how I could present the alert before dismissing the ViewController?

I use the below for the alert:

let alertView = UIAlertController(title: "Houps", message: "Could not connect to the server.", preferredStyle: .Alert)
alertView.addAction(UIAlertAction(title: "Ok", style: .Default, handler: nil))
presentViewController(alertView, animated: true, completion: nil)

EDIT: The code is as below:

if let httpResponse = response as? NSHTTPURLResponse {
                print("HTTP response: \(httpResponse.statusCode)")
                if httpResponse.statusCode == 201 {
                    taskViewController.dismissViewControllerAnimated(true, completion: nil)
                }
            } else {
                print("No HTTP response")
                let alertView = UIAlertController(title: "Houps", message: "Could not connect to the server.", preferredStyle: .Alert)
                alertView.addAction(UIAlertAction(title: "Ok", style: .Default, handler: nil))
                presentViewController(alertView, animated: true, completion: nil)

            }

You should call presentViewController on the taskViewController:ORKTaskViewController object

taskViewController.presentViewController(alertView, animated: true, completion: nil)