docs: def on_complete(cls, req):
          """
          Callback called when the request to REST is done. Handles the errors
          and if there is none, :class:`.OutputPicker` is shown.
          """
          # handle http errors
          if not (req.status == 200 or req.status == 0):
              ViewController.log_view.add(req.text)
              alert(req.text)  # TODO: better handling
              return
  
          try:
              resp = json.loads(req.text)
          except ValueError:
              resp = None
  
          if not resp:
              alert("Chyba při konverzi!")  # TODO: better
              ViewController.log_view.add(
                  "Error while generating MARC: %s" % resp.text
              )
              return
  
          OutputPicker.show(resp)