Sudden NullReferenceException in HandleErrorResponse when calling MessageResource.Create on production

[adamtal3]

Version: reproduced on 5.4.0 and 5.5.0

Code Snippet

            try
            {
                var msg = MessageResource.Create(new PhoneNumber(phoneNumber),
                    from: new PhoneNumber(alphanumericId),
                    body: messageBody);
            }
            catch (Twilio.Exceptions.RestException ex)
            {
                    // This doesn't catch the error (it's not a RestException)
                    throw;
            }

Exception / Log

System.NullReferenceException: Object reference not set to an instance of an object.
   at Twilio.Http.WebRequestClient.HandleErrorResponse(HttpWebResponse errorResponse)
   at Twilio.Http.WebRequestClient.MakeRequest(Request request)
   at Twilio.Clients.TwilioRestClient.Request(Request request)
   at Twilio.Rest.Api.V2010.Account.MessageResource.Create(CreateMessageOptions options, ITwilioRestClient client)
   at Twilio.Rest.Api.V2010.Account.MessageResource.Create(PhoneNumber to, String pathAccountSid, PhoneNumber from, String messagingServiceSid, String body, List`1 mediaUrl, Uri statusCallback, String applicationSid, Nullable`1 maxPrice, Nullable`1 provideFeedback, Nullable`1 validityPeriod, ITwilioRestClient client)	

Steps to Reproduce

I could reproduce this error only on my production server.
Running on localhost worked as expected.
I get this error sending to a valid number that works at first but after about 5 messages it stops working (with exact same to/from/body values). Also, it fails immediately (not a timeout).
It keeps failing with this exception until I restart the application pool or the iis server.

What can cause this NullReferenceException inside HandleErrorResponse?
I couldn't get any valuable information from this exception..

[adamtal3]

I've managed to find the problem using a custom build I made of this library. I've created a pull request fixing the exception handling so other users can see the problem right.

[codejudas]

@adamtal3 What was the root cause of the error? Anything we need to fix in the library itself apart from exposing more information when an error occurs?

[adamtal3]

@efossier The error was caused by an unsupported TLS security protocol (It was sudden because I had a task running once an hour that accesses an https api and sets the ServicePointManager.SecurityProtocol to a different value before doing so - and it seems it didn't set it back to its original value).

Once I've used my custom build I saw the correct error.. the main problem is that the error was hidden and disguised by NullReferenceError.
When I had an idea it was using the wrong TLS protocol I just added: (and I made sure the other place in my code also sets back the SecurityProtocol value to its original state)

    var securityProtocol = ServicePointManager.SecurityProtocol;
    var expect100Continue = ServicePointManager.Expect100Continue;

    try
    {
        ServicePointManager.Expect100Continue = true;
        ServicePointManager.SecurityProtocol |= SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;

        // ... MessageResource.Create ...
    }
    finally
    {
        ServicePointManager.Expect100Continue = expect100Continue;
        ServicePointManager.SecurityProtocol = securityProtocol;
    }