validateTenantId(tenantId) {
    const uiConfigs = this.config_.get('tenants');
    if (!uiConfigs ||
        (!uiConfigs.hasOwnProperty(tenantId) &&
         !uiConfigs.hasOwnProperty(
             UiHandlerConfig.ConfigKeys.DEFAULT_CONFIG_KEY))) {
      throw new Error('Invalid tenant configuration!');
    }
  }
Validates the tenant ID or the top-level project config key.
@param {string} tenantId The tenantId or top-level project config key.