From Apple Documentation:
nil cannot be used with nonoptional constants and variables. If a
constant or variable in your code needs to work with the absence of a
value under certain conditions, always declare it as an optional value
of the appropriate type
also very importante to notice:
Swift’s nil is not the same as nil in Objective-C. In Objective-C, nil
is a pointer to a nonexistent object. In Swift, nil is not a
pointer—it is the absence of a value of a certain type. Optionals of
any type can be set to nil, not just object types
I hope that helps you!