code: public CMASpaceMembership fetchOne(String spaceId, String membershipId) {
      assertNotNull(spaceId, "spaceId");
      assertNotNull(membershipId, "membershipId");
  
      return service.fetchOne(spaceId, membershipId).blockingFirst();
    }
docs: Fetches one space membership by its id from Contentful.
  
  @param spaceId      the space this membership is hosted by.
  @param membershipId the id of the membership to be found.
  @return null if no membership was found, otherwise the found membership.
  @throws IllegalArgumentException if space id is null.
  @throws IllegalArgumentException if membership id is null.