 
     typealias Pair = StateActionPair<UuidState<MainWindow.State>, MainWindow.Action>
 
+    init(_ baseServerUrl: URL) {
+      self.baseServerUrl = baseServerUrl
+    }
+
     func reduce(_ source: Observable<Pair>) -> Observable<Pair> {
       return source.map { pair in
-        guard let baseServerUrl = self.parentTransformer?.baseServerUrl else {
-          return pair
-        }
-
         var state = pair.state.payload
 
         switch pair.action {

  static let errorPath = "/tools/preview/error.html"
  static let nonePath = "/tools/preview/empty.html"

  let forMainWindow = MainWindowPreviewReducer()
  let forOpenedFileList = OpenedFileListReducer()

  init(baseServerUrl: URL) {
    self.baseServerUrl = baseServerUrl

    self.forMainWindow.parentTransformer = self
    self.forOpenedFileList.parentTransformer = self
  }

  fileprivate let baseServerUrl: URL
}

extension PreviewReducer {

            return pair
          }

          state.preview = PreviewUtils.state(for: pair.state.uuid, baseUrl: self.baseServerUrl, buffer: buffer)

        case .close:
          state.preview = PreviewUtils.state(for: .none, baseUrl: self.baseServerUrl)

        default:
          return pair
