Error Handling in Swift 3 -

i'm migrating code on swift 3 , see bunch of same warnings do/try/catch blocks. want check if assignment doesn't return nil , print out console if doesn't work. catch block says "is unreachable because no errors thrown in 'do' block". want catch errors 1 catch block.

let xmlstring: string?     do{         //warning line below: "no calls throwing function occurs within 'try' expression         try xmlstring = string(contentsofurl: accessurl, encoding: string.encoding.utf8)          var xmldict = xmldictionaryparser.sharedinstance().dictionary(with: xmlstring)         if let models = xmldict?["cygnet"] {             self.cygnets = models as! nsarray         }      //warning line below: "catch block unreachable because no errors thrown in 'do' block     } catch {         print("error getting xml string")     } 

how write proper try catch block handle assignment errors?

one way can throwing own errors on finding nil.

with having sort of own error:

enum myerror: error {     case foundnil(string) } 

you can write this:

    do{         let xmlstring = try string(contentsof: accessurl, encoding: string.encoding.utf8)         guard let xmldict = xmldictionaryparser.sharedinstance().dictionary(with: xmlstring) else {             throw myerror.foundnil("xmldict")         }         guard let models = xmldict["cygnet"] as? nsarray else {             throw myerror.foundnil("models")         }         self.cygnets = models     } catch {         print("error getting xml string: \(error)")     }