Unexpectedly found nil while unwrapping an Optional value in Swift 3 - swift3

Question

I have a timetable app, and after converting everything to Swift 3, one particular line threw an EXC_BAD_INSTRUCTION error, stating "Unexpectedly found nil while unwrapping an Optional value"
Here is the code, the final line returns the error:
class CyclicDay {
enum CyclicDayError: Error {
case invalidStartDate }
lazy var baseline: Date! = {
var components = DateComponents()
components.day = 27
components.month = 3
components.year = 2017
return Calendar.current.date(from: components)!
}()
func dayOfCycle(_ testDate: Date) throws -> Int {
if let start = baseline {
let interval = testDate.timeIntervalSince(start as Date)
let days = interval / (60 * 60 * 24)
return Int(days.truncatingRemainder(dividingBy: 14)) + 1 }
throw CyclicDayError.invalidStartDate }}
override func viewDidLoad() {
// Do any additional setup after loading the view, typically from a nib.
let cd = CyclicDay()
let day = try! cd.dayOfCycle(Date())
let date = Date()
let calendar = Calendar.current
let components = calendar.dateComponents([.hour, .minute], from: date)
let hour = components.hour
let minutes = components.minute
_ = "\(String(describing: hour)):\(String(describing: minutes))"
let lengthTestHour = "\(String(describing: hour))"
let lengthTestMinute = "\(String(describing: minutes))"
let formatter = DateFormatter()
formatter.dateFormat = "a"
formatter.amSymbol = "AM"
formatter.pmSymbol = "PM"
let dateString = formatter.string(from: Date())
var finalHour = String()
if lengthTestHour.characters.count == 1 {
finalHour = String("0\(String(describing: hour))")
} else {
finalHour = "\(String(describing: hour))"
}
if lengthTestMinute.characters.count == 1 {
_ = "0\(String(describing: minutes))"
} else {_ = minutes }
let convert = finalHour
let mTime = Int(convert)
// mTime * 100 + minutes
let compTime = mTime! * 100 + minutes!

Answer 1

In Swift 3 all date components are optional, you need to unwrap the optionals
let hour = components.hour!
let minutes = components.minute!
otherwise you get in trouble with the string interpolations.
Btw: You don't need String(describing just write for example
_ = "\(hour):\(minutes)"
I'm wondering anyway why you do all the formatting stuff manually instead of using the date formatter you created.

Answer 2

The problem lies in these two lines:
let lengthTestHour = "\(String(describing: hour))"
let lengthTestMinute = "\(String(describing: minutes))"
You thought lengthTestHour will store a value like "7" and lengthTestMinute will have a value like "33". But no, lengthTestHours actually holds "Optional(7)" and lengthTestMinutes actually holds "Optional(33)".
You then assign lengthTestHour to convert and try to convert that Optional(7) thing into an Int, which obviously can't be done. Now mTime is nil and you try to force unwrap in the last line. BOOM!
This is because String(describing:) returns an optional. The two lines can be shortened and fixed by doing:
let lengthTestHour = "\(hour!)"
let lengthTestMinute = "\(minute!)"