iOS back button working weirdly in iOS8

Question

This is something weird and happening randomly.

I am pushing a viewcontroller so by default ios adds the back button on the top left corner of the app. Whenever user presses that back button it behaves very weirdly. Attaching image of the same!

There is no way user can go back.It stops there(app doesn't hang)but back button stops working. I am using following two lines to display a new ViewController.

    var vc=ShowNotificationController()
    self.navigationController?.pushViewController(vc, animated: true)

I don't know why it is happening and it is happening very randomly.

We are using xcode 6.2 & swift