swift3 - Nil Error While Running -

i working on todo app,it completed , running begins give error "fatal error: unexpectedly found nil while unwrapping optional value".need guide!

class viewcontroller: uiviewcontroller, uitableviewdatasource, uitableviewdelegate{   @iboutlet weak var tableview: uitableview!  var tasks : [task] = [ ] override func viewdidload() {     super.viewdidload()     tableview.datasource = self     tableview.delegate = self     // additional setup after loading view, typically nib. }  override func viewwillappear(_ animated: bool) {     getdata()      tableview.reloaddata()  }   func tableview(_ tableview: uitableview, numberofrowsinsection section: int) -> int {     return tasks.count }  func tableview(_ tableview: uitableview, cellforrowat indexpath: indexpath) -> uitableviewcell {     let cell = uitableviewcell()      let task  = tasks[indexpath.row]      if task.isimportant{         cell.textlabel?.text = " ★   \(task.name!)"      }else{         cell.textlabel?.text = task.name!     }      return cell }  func getdata() {     let context =  (uiapplication.shared.delegate as! appdelegate).persistentcontainer.viewcontext      do{     tasks = try  context.fetch(task.fetchrequest())     }     catch {         print ("failed!")     } } 

}

you should avoid unwrapping optionals ! because of danger of encountering runtime error if optional missing. try following:

let taskname = task.name ?? "no name" if task.isimportant{     cell.textlabel?.text = " ★   \(taskname)" }else{     cell.textlabel?.text = taskname }