RxSwift) cannot bind rx to IBoutlet button, occurs "Thread 1: Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value"

Question

I want to bind that debounce thing to my UIButton-star, but there are some issues.

@IBOutlet weak var star: UIButton!

    
    private func starTapRx(){
        print( "rx" )     // this works well

//        star.rx.tap.asDriver().          <- same error here
//             debounce(.seconds(1)).drive(onNext: { (_) in
//                print("presssssss")
//             }).disposed(by: bag)
        
        
        star.rx.tap.bind{ print("bind")}.disposed(by: bag)
    }

this is the error:

It occurs on ".rx", and I cannot understand why it is Optional value. I import RxSwift, RxCocoa.