Graph API request - "object reference not set to an instance..."

Partially resolved, scroll down for the update...

I am trying to make simple call with Graph API; at the first try to just get channels in my team without the need of login manually by the user. So, looks like I need to use Ms Client ID and Client secret and some token. With my understanding I need to get the token from HTTP request (which is implemented in GraphAuthenticator.

The code that throws an exception (in func OnMessageActivityAsync):

var clientId = "<client id from app registration>";
var clientSecret = "<client secret value from app registration>";
GraphAuthenticator auth = new GraphAuthenticator(clientId, clientSecret);
var accessToken = auth.getAuthenticationToken();

var graphClient = new GraphClientService().GetAuthenticatedGraphClient(accessToken);
var team = graphClient.Teams["<my team id>"];
var channels = team.Channels;
var req = channels.Request();
^var reqTask = await req.GetAsync();
var resultList = new List<string>(reqTask.Count);

Second part has been divided to find exact null-reference (System.NullReferenceException thrown in the line with the declaration of reqTask), but looks like req is NOT a null, because I can read its members during debugging, like RequestUrl. GraphAuthenticator class simply make a HTTP request to obtain a token

var DATA = Encoding.ASCII.GetBytes($"grant_type=client_credentials&client_id={ MS_APP_ID_ENC }&client_secret={ MS_APP_PASS_ENC }&scope=https%3A%2F%2Fgraph.microsoft.com%2F.default");
//HTTP request with successfully acquiring token...

GraphClientService comes from examples in GitHub provided by Ms (IGraphClientService has this one method only).

    public class GraphClientService : IGraphClientService {
    public GraphServiceClient GetAuthenticatedGraphClient(string accessToken) {
        return new GraphServiceClient(new DelegateAuthenticationProvider(
            async (request) => {
                request.Headers.Authorization =
                    new AuthenticationHeaderValue("Bearer", accessToken);
                await Task.CompletedTask;
            }
        ));
    }
}

I was trying to obtain maybe some data of user, even trying SimpleGraphClient - exactly as it is in the samples.

var user = await graphClient.Me.Request().Select(u => new {
                                                            u.DisplayName,
                                                            u.Mail,
                                                            u.UserPrincipalName
                                                        }).GetAsync();

But result was the same. Null reference exception. Am I missing something? I am sure about credentials, as Graph send me the token. Details of the exception:

Callstack:

This exception was originally thrown at this call stack:
Microsoft.Graph.HttpProvider.SendAsync(System.Net.Http.HttpRequestMessage, System.Net.Http.HttpCompletionOption, System.Threading.CancellationToken)
System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(System.Threading.Tasks.Task)
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(System.Threading.Tasks.Task)
System.Runtime.CompilerServices.ConfiguredTaskAwaitable<TResult>.ConfiguredTaskAwaiter.GetResult()
Microsoft.Graph.BaseRequest.SendRequestAsync(object, System.Threading.CancellationToken, System.Net.Http.HttpCompletionOption)

Source: Microsoft.Graph.Core

StackTrace:

at Microsoft.Graph.HttpProvider.<SendAsync>d__18.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()
   at Microsoft.Graph.BaseRequest.<SendRequestAsync>d__35.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()
   at Microsoft.Graph.BaseRequest.<SendAsync>d__31`1.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()
   at Microsoft.Graph.TeamChannelsCollectionRequest.<GetAsync>d__4.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Microsoft.BotBuilderSamples.BatchBot`1.<OnMessageActivityAsync>d__6.MoveNext() in G:\...cs:line 90

TargetSite: void MoveNext()

Looking into req variable I saw there is no header (count = 0). Shouldn't be there a authorization header visible?

============================ @EDIT I decided to continue this question, even though we know the root problem of the exception.

But when I am reading docs and other forums looks like problematic Graph request should throw an ServiceException. In my code it is other exception somewhere deep in Graph code, I could not get there, looks like NuGet package doesn't have symbols.

Is it possible to read error message sent by Graph and avoid this exception being thrown? Surrounding my code with try doesn't make any difference (it's an async method).