swift - 为什么 Swift 5 的 safeAreaInserts 可选链在 window??.safeAreaInsets 中需要两个问号而不是一个问号？

Question

Swift 编译器抱怨

let insets : UIEdgeInsets? = UIApplication.shared.delegate?.window?.safeAreaInsets

带有错误消息

Value of optional type UIWindow? must be unwrapped to refer to member safeAreaInsets of wrapped base type UIWIndow. Chain the optional using '?' to access member safeAreaInsets only for non-nil base values.

编码

let insets : UIEdgeInsets? = UIApplication.shared.delegate?.window??.safeAreaInsets

工作正常。为什么必填后的第二个问号window？这里的两个问号似乎不构成 nil-coalescing 运算符。我显然错过了一些东西。