gistfile1.cs

using (var client = new ImapClient ()) {
    var inbox = client.Inbox;

    client.Connect(_currEmailAccountInfoAcct.Servername, 993, true, _emailCancellationTokenSource.Token);

    inbox.Open(FolderAccess.ReadOnly); // this throws NullReferenceException

    client.AuthenticationMechanisms.Remove("XOAUTH2");
    client.Authenticate(_currEmailAccountInfoAcct.UserName, _currEmailAccountInfoAcct.Password);

    const MessageSummaryItems summaryItems =  MessageSummaryItems.Envelope | 
          MessageSummaryItems.Flags | MessageSummaryItems.InternalDate |
          MessageSummaryItems.MessageSize | MessageSummaryItems.BodyStructure;

    var summaries = await inbox.Fetch(minIndex, maxIndex, summaryItems);

    client.Disconnect(true);
}