getTenantConfig_(tenantId) {
    this.validateTenantId(tenantId);
    const uiConfigs = this.config_.get('tenants');
    return uiConfigs[tenantId] ||
        uiConfigs[UiHandlerConfig.ConfigKeys.DEFAULT_CONFIG_KEY];
  }
Validates the tenant ID provided and returns the config object for the
specific tenant.
@param {string} tenantId The tenantId or top-level project config key.
@return {!Object} The config object of the tenant.
@private