Swift委托调用致命错误

protocol Delegate {
    func requiredMethod()
}

class DelegateObject: Delegate {
    func requiredMethod() {
        print("Required method called")
    }
}

class MainObject {
    var delegate: Delegate?
    
    func callDelegateMethod() {
        if let delegate = delegate {
            delegate.requiredMethod()
        } else {
            print("Delegate object not set")
        }
    }
}

let mainObject = MainObject()
mainObject.callDelegateMethod() // 输出 "Delegate object not set"

let delegateObject = DelegateObject()
mainObject.delegate = delegateObject
mainObject.callDelegateMethod() // 输出 "Required method called"