getAdminRestrictedOperationAdminEmail() {
    const adminRestrictedOperation =
        this.config_.get('adminRestrictedOperation');
    if (adminRestrictedOperation && adminRestrictedOperation['adminEmail']) {
      return /** @type {string} */(adminRestrictedOperation['adminEmail']);
    }
    return null;
  }
@return {?string} The admin contact email when users are restricted from
    sign up. Otherwise, null is returned.