Export mailbox backup file gets stuck when sharing prompt need to be presented on the iPad.

The UIActivityViewController's has non-null popoverPresentationController property when running on iPad. So, try below.

`if let wPPC = activityVC.popoverPresentationController {
    wPPC.sourceView = some view
    //  or
    wPPC.barButtonItem = some bar button item
}
presentViewController( activityVC, animated: true, completion: nil )`