Utilicé algún código fuente:

 KGModalContainerView *containerView = 
     self.containerView = 
         [[KGModalContainerView alloc] initWithFrame:containerViewRect];

Me da :Assigning retained object to weak property; object will be released after assignment

editar:

@interface KGModal()
  @property (strong, nonatomic) UIWindow *window;
  @property (weak, nonatomic) KGModalViewController *viewController;
  @property (weak, nonatomic) KGModalContainerView *containerView;
  @property (weak, nonatomic) UIView *contentView;
@end

KGModalContainerView *containerView = 
    self.containerView = 
        [[KGModalContainerView alloc] initWithFrame:containerViewRect];
containerView.modalBackgroundColor = self.modalBackgroundColor;
containerView.autoresizingMask = UIViewAutoresizingFlexibleLeftMargin |
                                 UIViewAutoresizingFlexibleRightMargin |
                                 UIViewAutoresizingFlexibleTopMargin |
                                 UIViewAutoresizingFlexibleBottomMargin;
containerView.layer.rasterizationScale = [[UIScreen mainScreen] scale];
contentView.frame = (CGRect){padding, padding, contentView.bounds.size};
[containerView addSubview:contentView];
[viewController.view addSubview:containerView];