title: UIKit Dynamics "SolveVelocityConstraints" crash
body: <p>I keep getting a crash when using <code>UIKit Dynamics</code>. It doesn't happen every time the code runs, but it happens fairly often. Has anyone come across this?</p>
  
  <pre><code>Assertion failed: (a.x &gt;= 0.0f &amp;&amp; a.y &gt;= 0.0f), 
  function SolveVelocityConstraints, file 
  /SourceCache/PhysicsKit/PhysicsKit-
  4.6/PhysicsKit/Box2D/Dynamics/Contacts/b2ContactSolver.cpp, line 422.
  </code></pre>
  
  <p>This is the code that generates the error. I'm <em>guessing</em> it has something to do with the collision boundaries.</p>
  
  <pre><code>UIDynamicAnimator *animator = 
   [[UIDynamicAnimator alloc] initWithReferenceView:self];
  
  UIGravityBehavior *gravityBehavior = 
   [[UIGravityBehavior alloc] initWithItems:@[self.rewardButton]];
  gravityBehavior.gravityDirection = CGVectorMake(0.0, 1.0);
  gravityBehavior.magnitude = 1.0f;
  
  UICollisionBehavior* collisionBehavior = 
   [[UICollisionBehavior alloc] initWithItems:@[self.rewardButton, 
                                                self.processingView]];
  collisionBehavior.collisionDelegate = self;
  UIEdgeInsets insets = UIEdgeInsetsMake(-700, 60, 200, 60);
  [collisionBehavior setTranslatesReferenceBoundsIntoBoundaryWithInsets:insets];
  
  UIDynamicItemBehavior* propertiesBehavior = 
   [[UIDynamicItemBehavior alloc] initWithItems:@[self.rewardButton]];
  float elasticity = arc4random() % 4 * 0.1 + 0.1f;
  propertiesBehavior.elasticity = elasticity;
  
  [animator addBehavior:propertiesBehavior];
  [animator addBehavior:gravityBehavior];
  [animator addBehavior:collisionBehavior];
  
  self.animator = animator;
  </code></pre>