UIVisualEffectView添加子视图崩溃

iOS11及以上版本，UIVisualEffectView 直接addSubView会崩溃。
如果需要添加，应使用view.contentView addSubView

[self.blurView.contentView addSubview:_exitImageView];

Apple官方给出的解释是：
Masks applied to the UIVisualEffectView itself are forwarded to all internal views, You are free to apply a mask to just the contentView.The mask you provide to UIVisualEffectView will not be the view that actually performs the mask.
在UIVisualEffectView上添加的view,在离屏渲染后,可能会和最终展示的view有出入,而且会影响所有UIVisualEffectView上的子view,这并不是我们想要的结果