ios - What have I missed ?? fatal error unexpected nil found while unwrapping -

@iboutlet var txtusername: uitextfield! @iboutlet var txtpassword: uitextfield!  @ibaction func signintapped(sender: uibutton) {    var username:nsstring = txtusername.text     var password:nsstring = txtpassword.text   if (self.txtusername.text.isempty)     {          (uialertview(title: "fields cant empty", message: "please enter email id , password",             delegate: nil, cancelbuttontitle: "ok").show())         return     }     else if (range==nil)     {         (uialertview(title: "invalid email id", message: "please enter correct email id", delegate: nil, cancelbuttontitle: "ok").show())         return     }     else if (txtpassword.text == ""  )     {          var alertview:uialertview = uialertview()         alertview.title = "failed!"         alertview.message = "please enter password"         alertview.delegate = self         alertview.addbuttonwithtitle("ok")         alertview.show()         return     }     else     {         var url:nsurl = nsurl(string:"http://192.168.0.31:8097/api/user/getvalidateuser" + "/" + username + "/" + txtpassword.text)!          var err:nserror?         var request:nsmutableurlrequest = nsmutableurlrequest(url: url)         request.httpmethod = "get"         request.addvalue("application/json", forhttpheaderfield: "content-type");         request.addvalue("application/json", forhttpheaderfield: "accept");         var reponseerror: nserror?         var response: nsurlresponse?         var urldata: nsdata? = nsurlconnection.sendsynchronousrequest(request, returningresponse:&response, error:&reponseerror)           if ( urldata != nil )         {             let res = response nshttpurlresponse!;              var responsedata:nsstring  = nsstring(data:urldata!, encoding:nsutf8stringencoding)!             nslog("response code: %ld", res.statuscode);             //var   message :string = "valid user"             if (res.statuscode == 200)             {                 if(responsedata != "\"valid user\"")                 {                     var alertview:uialertview = uialertview()                     alertview.title = "failed"                     alertview.message = responsedata                     alertview.delegate = self                     alertview.addbuttonwithtitle("ok")                     alertview.show()                     return;                 }                 //get address book reference                 //gabberscontroller().getaddressbook()                  //  var alertview:uialertview = uialertview()                 //   alertview.title = "sign in message"                 //  alertview.message = responsedata                 //  alertview.delegate = self                 //  alertview.addbuttonwithtitle("ok")                 //  alertview.show()             }              else {                 var alertview:uialertview = uialertview()                 alertview.title = "failed"                 alertview.message = responsedata;                 if let error = reponseerror {                     alertview.message = (error.localizeddescription)                 }                 alertview.delegate = self                 alertview.addbuttonwithtitle("ok")                 alertview.show()             }           }          else {             var alertview:uialertview = uialertview()             alertview.title = "failed"             alertview.message = "connection failure";             if let error = reponseerror {                 alertview.message = (error.localizeddescription)             }             alertview.delegate = self             alertview.addbuttonwithtitle("ok")             alertview.show()           }         //self.performseguewithidentifier:@"goto__gabbers" sender:self;      }   } 

for above code got sudden error, thought may missed reference outlet deleted existing 1 , created again in story board when run application same error throwing not getting missed here. executed @ first time further not,throwing fatal unexpected nil error can tell missed here.

this exact error getting:

fatal error: unexpectedly found nil while unwrapping optional value

posted full code

as per swift1.2 can below.

@iboutlet var txtusername: uitextfield! @iboutlet var txtpassword: uitextfield!  @ibaction func signintapped(sender: uibutton) {    if let username = txtusername.text as? string, let password = txtpassword.text as? string {      var url:nsurl = nsurl(string:"http://192.168.0.31:8097/api/user/getvalidateuser" + "/" + username + "/" + password)!     var err:nserror?    } }