Publish Provider Hosted App Problem

I have a default provider-hosted application. when i publish it on rackspace server i got this error:

System.NullReferenceException: Object reference not set to an instance of an object. at Portal.TokenHelper.GetAccessToken(String refreshToken, String targetPrincipalName, String targetHost, String targetRealm) at Portal.SharePointAcsContext.RenewAccessTokenIfNeeded(Tuple`2& accessToken, Func`1 tokenRenewalHandler) at Portal.SharePointAcsContext.GetAccessTokenString(Tuple`2& accessToken, Func`1 tokenRenewalHandler)

My code:

private void GetSPUserDetails(HttpContextBase httpContextBase, dynamic viewBag)
{
            var spContext = SharePointContextProvider.Current.GetSharePointContext(httpContextBase);

            // Store SPHostUrl in ViewBag
            viewBag.SPHostUrl = spContext.SPHostUrl.AbsoluteUri.TrimEnd('/');

            using (var clientContext = spContext.CreateUserClientContextForSPHost())
            {
                if (clientContext != null)
                {
                    var user = UserHelper.GetCurrentUserInfo(clientContext);

                    // Store typically used variables in ViewBag
                    viewBag.WebTitle = user.WebTitle;
                    viewBag.UserLogin = user.Email;
                    viewBag.UserName = user.Title;
                }
            }
}

I'm using WebDeploy to publish my portal.

It's a provider-hosted app for SharePoint Online (o365) using VS2013

can anyone help me on this?

Thanks for your time.