How do I fix UIPopoverPresentationController issue for iPad app?

Question

I'm developing an iPad app, and when I run it in the simulator and hit the share button (bar button item), I have got the following error message (I did not have that issue with the iPhone app). How can I fix it?

Terminating app due to uncaught exception 'NSGenericException', reason: 'UIPopoverPresentationController (<_UIAlertControllerActionSheetRegularPresentationController: 0x7bfa3c90>) should have a non-nil sourceView or barButtonItem set before the presentation occurs.'

My code is the following (the top 3 lines are in ViewDidLoad):

    self.navigationItem.titleView = titleButton
    let shareButton: UIBarButtonItem = UIBarButtonItem(image: UIImage(named: "ShareIcon"), style: UIBarButtonItemStyle.Plain, target: self, action: "shareButtonPressed:")
    self.navigationItem.rightBarButtonItem = shareButton

}

func shareButtonPressed(sender: AnyObject) {
    NSLog("shareButton pressed")
    let stringtoshare: String = ""

    //add current screen shot image to share
    let imagetoshare = captureScreen()

    let activityItems: [AnyObject] = [stringtoshare, imagetoshare]
    let activityVC: UIActivityViewController = UIActivityViewController(activityItems: activityItems, applicationActivities: nil)
    activityVC.excludedActivityTypes = [UIActivityTypeAssignToContact, UIActivityTypePrint, UIActivityTypePostToWeibo]
    self.presentViewController(activityVC, animated: true, completion: { _ in })

}

func captureScreen() -> UIImage {

    UIGraphicsBeginImageContextWithOptions(self.view.bounds.size, false, 0);
    self.view.drawViewHierarchyInRect(view.bounds, afterScreenUpdates: true)
    let image:UIImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return image
}