EXC_BAD_INSTRUCTION error for checking if xml element is empty in swift

I'm having problem checking if the xml element has desire tag or not :

I wrote my code like below :

//checking if link is empty
if xml["rss"]["channel"]["item"][index]["source"].element!.attributes["url"] != "" {
   Link = (xml["rss"]["channel"]["item"][index]["source"].element?.attributes["url"])!
 } else {
     Link = ""
 }

When I run this code i get error of : Thread3 EXC_BAD_INSTRUCTION , for the first line of my code ,but when I run the code like below : it goes through if and does not understand that source element is not exist at all! code :

 //checking if link is empty
  if (xml["rss"]["channel"]["item"][index]["source"].element?.attributes["url"] as String?) != nil {
     Link = (xml["rss"]["channel"]["item"][index]["source"].element?.attributes["url"] as String?)!
   } else {
         Link = ""
   }

How check if source element is exist or not in swift 2 ?