Passing string causing crash sometimes in swift - ios

Question

I run into strange thing I'm getting a crash it this function:
func addQuestion(text:String!)
{
if let txt=text
{
let question=PFObject(className: "questions")
question["text"]=txt
question["userAdding"]=PFUser.currentUser()
question["answeredCount"]=0
question.setObject([], forKey: "answers")
question.setObject(NSDate(), forKey: "dateAdded")
question.setObject(Int(arc4random()) % 10, forKey: "random")
if debug
{question.saveInBackgroundWithBlock(nil)}
else
{question.saveEventually()}
}
}
I'm getting a crash in "if let txt=text"
here is a back trace:
thread #1: tid = 0x241c0c, 0x00063470 pytajnik`pytajnik.ApiClient.addQuestion (text=Some, self=(ObjectiveC.NSObject = {}, debug = true))(Swift.ImplicitlyUnwrappedOptional<Swift.String>) -> () + 10796 at ApiClient.swift:32, queue = 'com.apple.main-thread', stop reason = EXC_BREAKPOINT (code=EXC_ARM_BREAKPOINT, subcode=0xe7ffdefe)
frame #0: 0x00063470 pytajnik`pytajnik.ApiClient.addQuestion (text=Some, self=(ObjectiveC.NSObject = {}, debug = true))(Swift.ImplicitlyUnwrappedOptional<Swift.String>) -> () + 10796 at ApiClient.swift:32
frame #1: 0x00035428 pytajnik`pytajnik.AddQuestionViewController.addQuestion (sender=AnyObject at 0x006ca054, self=0x17ef85d0)(Swift.AnyObject) -> () + 796 at AddQuestionViewController.swift:17
frame #2: 0x00035a6c pytajnik`#objc pytajnik.AddQuestionViewController.addQuestion (pytajnik.AddQuestionViewController)(Swift.AnyObject) -> () + 100 at AddQuestionViewController.swift:0
frame #3: 0x2a5fdc2a UIKit`-[UIApplication sendAction:to:from:forEvent:] + 70
frame #4: 0x2a5fdbd0 UIKit`-[UIControl sendAction:to:forEvent:] + 44
...
The thing is, the app is not crashing always. Sometime it works sometime it doesn't. And I could't find a pattern.
Could somebody help me understand what is wrong?
UPDATE
thread #1: tid = 0x243f69, 0x0005d478 pytajnik`pytajnik.ApiClient.addQuestion (text=Some, self=(ObjectiveC.NSObject = {}, debug = true))(Swift.Optional<Swift.String>) -> () + 9820 at ApiClient.swift:35, queue = 'com.apple.main-thread', stop reason = EXC_BREAKPOINT (code=EXC_ARM_BREAKPOINT, subcode=0xe7ffdefe)
frame #0: 0x0005d478 pytajnik`pytajnik.ApiClient.addQuestion (text=Some, self=(ObjectiveC.NSObject = {}, debug = true))(Swift.Optional<Swift.String>) -> () + 9820 at ApiClient.swift:35
frame #1: 0x0002f80c pytajnik`pytajnik.AddQuestionViewController.addQuestion (sender=AnyObject at 0x006c4054, self=0x166a5560)(Swift.AnyObject) -> () + 1760 at AddQuestionViewController.swift:17
frame #2: 0x0002fe40 pytajnik`#objc pytajnik.AddQuestionViewController.addQuestion (pytajnik.AddQuestionViewController)(Swift.AnyObject) -> () + 100 at AddQuestionViewController.swift:0
frame #3: 0x2a5fdc2a UIKit`-[UIApplication sendAction:to:from:forEvent:] + 70
frame #4: 0x2a5fdbd0 UIKit`-[UIControl sendAction:to:forEvent:] + 44
frame #5: 0x2a5e8862 UIKit`-[UIControl _sendActionsForEvents:withEvent:] + 582
frame #6: 0x2a5fd63c UIKit`-[UIControl touchesEnded:withEvent:] + 588
frame #7: 0x2a5fd316 UIKit`-[UIWindow _sendTouchesForEvent:] + 522
frame #8: 0x2a5f6be0 UIKit`-[UIWindow sendEvent:] + 544

Answer 1

You declared the function parameter as implicitly unwrapped optional !, meaning you are sure it will never be nil.
If its possible that this parameter can be nil declare it as optional:
func addQuestion(text: String?) {
if let txt = text {
...
}
}

Answer 2

You have declared the text parameter as an implicitly unwrapped optional, which causes the app to crash if text is nil, and this is what's happening in your case.
Unless you have a specific reason to use an implicitly unwrapped optional, it's always better to use an optional String?, or a non optional String if the parameter can never be nil.
So to fix the problem just change your function signature to this:
func addQuestion(text:String?)
Suggested reading: Implicitly Unwrapped Optionals