UIDynamicAnimator - won't work with UIView but will with Self.View?

It appears from the exception that self.gameOverFalling does not descend from finalScoreView in your view hierarchy. See the quote from UIDynamicAnimator class reference:

To animate views, create an animator with the initWithReferenceView: method. The coordinate system of the reference view serves as the coordinate system for the animator’s behaviors and items. Each dynamic item you associate with this sort of animator must be a UIView object and must descend from the reference view.

I solved this by adding to subview before adding any behaviour.

addSubview(square)
var gravity = UIGravityBehavior(items: [square])
gravity.magnitude = gravitySpeed
animator.addBehavior(gravity)