code: public CMAWebhook fetchOne(String spaceId, String webhookId) {
      assertNotNull(spaceId, "spaceId");
      assertNotNull(webhookId, "webhookId");
  
      return service.fetchOne(spaceId, webhookId).blockingFirst();
    }
docs: Retrieve exactly one webhook, whose id you know.
  <p>
  This method will override the configuration specified through
  {@link CMAClient.Builder#setSpaceId(String)} and will ignore
  {@link CMAClient.Builder#setEnvironmentId(String)}.
  
  @param spaceId   The id of the space to be hosting this webhook.
  @param webhookId The id of the webhook to be returned.
  @return The webhook found, or null, if no such webhook is available.
  @throws IllegalArgumentException if spaceId is null.
  @throws IllegalArgumentException if webhookId is null.