if you still want to keep the interface, just put a default inside the interface. like this

interface BootstrapListener {
    fun onBootstrapComplete()
    
    object DoNothing : BootstrapListener {
        override fun onBootstrapComplete() {}
    }
}

class PlayoutSocketClient(..., private val bootstrapListener: BootstrapListener = BootstrapListener.DoNothing)

You could instead, just do this:

interface BootstrapListener {

  fun onBootstrapComplete() = Unit
  fun someOtherMethod() = Unit
}