Null Reference Exception: Object reference not set to an instance of an object

I am using version 5.1.0 and still receiving the Null Reference Exception on SendMutationAsync and SendQueryAsync as well.
Appreciate your inputs ..
thank you!

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  | Name | Value | Type
-- | -- | -- | --
◢ | $exception | {System.NullReferenceException: Object reference not set to an instance of an object. at GraphQL.Client.Http.GraphQLHttpClientOptions.<>c.<.ctor>b__44_2(HttpResponseMessage r) in /_/src/GraphQL.Client/GraphQLHttpClientOptions.cs:line 60 at GraphQL.Client.Http.GraphQLHttpClient.SendHttpRequestAsync[TResponse](GraphQLRequest request, CancellationToken cancellationToken) in /_/src/GraphQL.Client/GraphQLHttpClient.cs:line 124 at GraphQL.Client.Http.GraphQLHttpClient.SendQueryAsync[TResponse](GraphQLRequest request, CancellationToken cancellationToken)

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  | Name | Value | Type
-- | -- | -- | --
  | StackTrace | " at GraphQL.Client.Http.GraphQLHttpClientOptions.<>c.<.ctor>b__44_2(HttpResponseMessage r)\n at GraphQL.Client.Http.GraphQLHttpClient.d__28`1.MoveNext()\n at GraphQL.Client.Http.GraphQLHttpClient.d__23`1.MoveNext()\n at VectorLMS.Person.d__2.MoveNext() in