swift - Terminating app due to uncaught exception 'NSUnknownKeyException' -

i'm trying built simple calculator , got exception. has clue?

code:

class viewcontroller: uiviewcontroller {     @iboutlet weak var display: uilabel!        var userisinthemiddleoftyping = false     @ibaction func touchdigit(sender: uibutton)     {         let digit = sender.currenttitle!         if userisinthemiddleoftyping         {             let textcurrentlyindisplay = display.text!             display.text = textcurrentlyindisplay + digit         }         else         {             display.text = digit         }         userisinthemiddleoftyping = true      } } 

go viewcontroller scene on left -> right click on "view controller" -> check if outlets links each others (if 1 have warning symbol, delete it) -> rebuild projects