I am parsing json in a function call. Most of the time the function gets executed but after checking my app crash report it is observed that it crashes at particular line sometimes. It is a rare crash but I am uncertain what makes it behave like that. Being amateur developer, I cannot get to exact solution, check out if you can?

func doThis() {  //url     let url = URL(string: “URL”)      var request = URLRequest(url: url!)      //httpmethod     request.httpMethod = "GET"      //session      let session = URLSession.shared      //task      let task = session.dataTask(with: request, completionHandler:     {(data: Data?, response: URLResponse?,error: Error?) -> Void in          if error != nil         {             print("unable to fetch")         }          do         {             //crashes on below line             let rootDic = try! JSONSerialization.jsonObject(with: data!, options: JSONSerialization.ReadingOptions.mutableContainers) as! NSDictionary              //crashes at above line.          -JsonParsing-         }         catch{             print("error")         }      }     )     //resumetask     task.resume()  } 

Is it because of mobile network issues or low speed of internet on user side? Or threading mistake by me?