code: public static StitchAppClient getAppClient(
        @Nonnull final String clientAppId
    ) {
      ensureInitialized();
  
      synchronized (Stitch.class) {
        if (!appClients.containsKey(clientAppId)) {
          throw new IllegalStateException(
              String.format("client for app '%s' has not yet been initialized", clientAppId));
        }
        return appClients.get(clientAppId);
      }
    }
docs: Gets an app client by its client app id if it has been initialized; throws if none can be
  found.
  
  @param clientAppId the client app id of the app client to get.
  @return the app client associated with the client app id.