You can’t safely call a method on a nil value in Swift. You need to explicitly force that in your code with an exclamation point to assume that the variable is not nil.
Looks like the server passed empty value in JSON which got converted to NSNull in Objective-C. It's a common mistake to assume null is returned instead of NSNull.
The Swift equivalent of the JSON library should return nil which you then need to explicitly check for non nilness before using unless you use the unsafe exclamation pint.
