WatchKit SMS with preset body

Question

I found this question which should have helped me, but the solution there is not working for me, and I am not sure if something has changed or if the problem is with my code.

let messageBody = "hello"
let urlSafeBody = messageBody.stringByAddingPercentEncodingWithAllowedCharacters(NSCharacterSet.URLHostAllowedCharacterSet())
print("URLSAFEBODY: \(urlSafeBody)")
WKExtension.sharedExtension().openSystemURL(NSURL(string: "sms:&body=\(urlSafeBody)")!)

When this code is executed, I get the message that optional urlSafeBody was force unwrapped while nil, leading to a crash. Why is urlSafeBody nil? I know that I'm force unwrapping it, but I don't understand why it is ever nil after being assigned explicitly.