Swift: Custom ViewController initializers

Solved it! One has to call the designated initializer which in this case is the init with nibName, obviously …

init(leftVC:UIViewController, rightVC:UIViewController, gap:Int)
{
    self.leftVC = leftVC
    self.rightVC = rightVC
    self.gap = gap

    super.init(nibName: nil, bundle: nil)

    setupScrollView()
    setupViewControllers()
}