Mysterious EXC_BAD_ACCESS in Swift playground

Swift_EXC_BAD_ACCESS.playground

//: Playground - noun: a place where people can play

import UIKit

protocol Base {
    
    static var key: String { get }
}

protocol BaseChild: Base {
    
}

struct ChildClass: BaseChild {
    
    static var key: String {
        return "key"
    }
}

class Worker {
    
    static var defaultWorker: Worker?
    
    func work<T: Base>(entity: T) {
        print(T.key)
    }
}

class ChildWorker: Worker {
    
    override func work<T: BaseChild>(entity: T) {
        print(T.key)
    }
}

extension Base {
    
    func work() {
        Worker.defaultWorker?.work(entity: self)
    }
}

class Test {
    
    static func run() {
        let object = ChildClass()
        let worker = ChildWorker()
        worker.work(entity: object)
        
        Worker.defaultWorker = worker
        object.work()
    }
}

Test.run()