code: public CMAUiExtension fetchOne(String spaceId, String environmentId, String extensionId) {
      assertNotNull(spaceId, "spaceId");
      assertNotNull(environmentId, "spaceId");
      assertNotNull(extensionId, "extensionId");
  
      return service.fetchOne(spaceId, environmentId, extensionId).blockingFirst();
    }
docs: Fetch one extension from the given space.
  <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 this is valid on.
  @param environmentId the id of the environment this is valid on.
  @param extensionId   the id of the extension to be fetched.
  @return the ui extension for a specific space.
  @throws IllegalArgumentException        if spaceId is null.
  @throws IllegalArgumentException        if environmentId is null.
  @throws IllegalArgumentException        if extensionId is null.
  @throws CMANotWithEnvironmentsException if environmentId was set using
  {@link CMAClient.Builder#setEnvironmentId(String)}.