解决方法
From Apple Documentation:
nil cannot be used with nonoptional constants and variables. If a@H_502_10@ constant or variable in your code needs to work with the absence of a@H_502_10@ value under certain conditions,always declare it as an optional value@H_502_10@ of the appropriate type
也非常重要的注意到：
Swift’s nil is not the same as nil in Objective-C. In Objective-C,nil@H_502_10@ is a pointer to a nonexistent object. In Swift,nil is not a@H_502_10@ pointer—it is the absence of a value of a certain type. Optionals of@H_502_10@ any type can be set to nil,not just object types
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