code: public static StitchAppClient initializeDefaultAppClient(
        @Nonnull final String clientAppId,
        @Nonnull final StitchAppClientConfiguration config
    ) {
      ensureInitialized();
      if (clientAppId == null || clientAppId.isEmpty()) {
        throw new IllegalArgumentException("clientAppId must be set to a non-empty string");
      }
  
      final StitchAppClient client;
      synchronized (Stitch.class) {
        if (defaultClientAppId != null) {
          throw new IllegalStateException(
              String.format(
                  "default app can only be set once; currently set to '%s'", defaultClientAppId));
        }
        client = initializeAppClient(clientAppId, config);
        defaultClientAppId = clientAppId;
      }
      return client;
    }
docs: Initializes the default app client for Stitch to use when using {@link
  Stitch#getDefaultAppClient()}. Can only be called once.
  
  @param clientAppId the client app id to initialize an app client for.
  @param config the configuration to use to build the app client.
  @return the app client that was just initialized.