Custom view failed to render instance of the agent - crashes when setting a UIImage

I have a custom view with a button. When adding the view in storyboard i get an error:

failed to render instance of the agent

When i debug the view from interface builder it crashes when it sets the image of the button.

CODE:

    @IBDesignable
class UIHeader: UIView {

private lazy var backButton: UIButton = {
    let btn = UIButton()
    btn.tintColor = UIColor.lightGrayColor()
    btn.translatesAutoresizingMaskIntoConstraints = false

    //CRASH HERE//
    btn.setImage(UIImage(named: "prev")!.imageWithRenderingMode(.AlwaysTemplate), forState: .Normal)


    btn.addTarget(self, action: #selector(UIHeader.OnBackButtonClickLister(_:)), forControlEvents: .TouchUpInside)
    return btn
}()


override init(frame: CGRect) {
    super.init(frame: frame)
    setupView()
}

required init?(coder aDecoder: NSCoder) {
    super.init(coder: aDecoder)
    setupView()
}
} 



extension UIHeader {

@IBInspectable
var backButtonImage: UIImage? {
    get {
        return backButton.imageForState(.Normal)
    }
    set (newImage) {
        backButton.setImage(newImage?.imageWithRenderingMode(.AlwaysTemplate), forState: .Normal)
    }
}
} 

extension UIHeader {
private func setupView() {
    translatesAutoresizingMaskIntoConstraints = false
    addSubview(backButton)
}
}