RxBroadcastReceiver + DownloadManager example

DowloadJob.kt

fun dowloadFile(url: String): Single<Long> {
  val expectedId: Long = DownloadManager.Request(Uri.parse(builder.apkUrl)).run { downloadManager.enqueue(this) }
  return downloadReceiver.map {
            it.getLongExtra(DownloadManager.EXTRA_DOWNLOAD_ID, -1)
        }.takeWhile {
            it == expectedId
        }.firstOrError().timeout(3, TimeUnit.MINUTES)
}

RxBroadcastReceiver.kt

class RxBroadcastReceiver(private val context: Context, private val filter: IntentFilter) :
    Observable<Intent>() {

    override fun subscribeActual(observer: Observer<in Intent>) {
        object : BroadcastReceiver() {
            override fun onReceive(context: Context, intent: Intent?) {
                intent?.let {
                    if (filter.matchAction(it.action)) {
                        observer.onNext(it)
                    }
                }
            }
        }.apply {
            observer.onSubscribe(ReceiverDisposable(context, this))
            context.registerReceiver(this, filter)
        }
    }

    private class ReceiverDisposable(
        private val context: Context,
        private val receiver: BroadcastReceiver
    ) : Disposable {

        private var isDisposed = false

        @Synchronized
        override fun isDisposed(): Boolean {
            return isDisposed
        }

        @Synchronized
        override fun dispose() {
            context.unregisterReceiver(receiver)
            isDisposed = true
        }
    }
}

Sometimes, I get an exception on unregisterReceiver when the receiver is already unregistered. Any idea on what causes it? Any suggestions on the disposing logic?