code: @Handler
      public void onRequestCompleted(
              Request.In.Completed event, IOSubchannel appChannel)
              throws InterruptedException {
          final Request.In requestEvent = event.event();
          // A check that also works with null.
          if (Boolean.TRUE.equals(requestEvent.get())
              || requestEvent.httpRequest().response().map(
                  response -> response.statusCode() != HttpStatus.NOT_IMPLEMENTED
                      .statusCode())
                  .orElse(false)) {
              // Some other component has taken care
              return;
          }
  
          // Check if "Not Found" should be sent
          if (providedFallbacks != null
              && providedFallbacks.contains(requestEvent.getClass())) {
              ResponseCreationSupport.sendResponse(
                  requestEvent.httpRequest(), appChannel, HttpStatus.NOT_FOUND);
              return;
          }
  
          // Last resort
          ResponseCreationSupport.sendResponse(requestEvent.httpRequest(),
              appChannel, HttpStatus.NOT_IMPLEMENTED);
      }
docs: Checks whether the request has been handled (value of {@link Request}
  event set to `true`) or the status code in the prepared response
  is no longer "Not Implemented". If not, but a fall back has been set,
  send a "Not Found" response. If this isn't the case either, send
  the default response ("Not implemented") to the client.
  
  @param event
  the request completed event
  @param appChannel the application channel
  @throws InterruptedException if the execution was interrupted