title: UIViewPropertyAnimator stops working after presenting another controller
body: <p>I use <code>UIViewPropertyAnimator</code> for shrinking my view controller’s header:</p>
  
  <pre><code>- (UIViewPropertyAnimator *)headerAnimator {
      if (!_headerAnimator) {
          _headerAnimator = [[UIViewPropertyAnimator alloc] initWithDuration:2.f curve:UIViewAnimationCurveLinear animations:^{
              self.profileImageBorderWidthConstraint.constant = 58.f;
              self.profileImageBorderHeightConstraint.constant = 58.f;
              self.profileImageBorder.layer.cornerRadius = 29.f;
  
              self.profileImageView.layer.cornerRadius = 25.f;
  
              self.nameLabelTopConstraint.constant = 16.f;
  
              [self.headerView layoutIfNeeded];
          }];
      }
  
      return _headerAnimator;
  }
  </code></pre>
  
  <p>On scrolling, I set the <code>fragment</code>:</p>
  
  <pre><code>- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
      CGFloat fraction = (scrollView.contentOffset.y / (HeaderHeight-CollapsedHeaderHeight));
      self.headerAnimator.fractionComplete = fraction;
  }
  </code></pre>
  
  <p>This works fine. But after I present an another <code>UIViewController</code> from this controller and dismiss it, the animator gets in an inactive state and stops responding to the fraction settings. How can I restart it?</p>
  
  <p>What I’ve already tried:</p>
  
  <ul>
  <li>setting my animator property to <code>nil</code>, to let recreate it next time (crashes)</li>
  <li>pausing it on <code>-viewWillAppear:</code> to become active (becomes active, but does not animate)</li>
  <li>start it on <code>-viewWillAppear:</code> (remains inactive)</li>
  </ul>