Microsoft.Bot.Builder - "Object reference not set to an instance of an object" about botbuilder-v3 HOT 5 CLOSED

using System;
using System.Threading.Tasks;
using KusBot.Common;
using KusBot.Forms;
using Kusto.Cloud.Platform.Utils;
using Microsoft.Bot.Builder.Dialogs;
using Microsoft.Bot.Builder.FormFlow;

namespace KusBot.Dialogs
{
    [Serializable]
    public class FormDialog : IDialog<string>
    {
        private readonly IDialog<RequestFormBase> m_requestForm;

        public FormDialog(IDialog<RequestFormBase> requestForm)
        {
            m_requestForm = requestForm;
        }

        public async Task StartAsync(IDialogContext context)
        {
            try
            {
                context.Call(m_requestForm, CompleteForm);
            }
            catch (Exception exception)
            {
                DiagnosticsTrace.TraceException($"Failure to complete request form.", exception, PrivateTracer.Tracer);
                await context.PostAsync(Prompts.FormExceptionMessage);
                context.Done(string.Empty);
            }
        }

        public async Task CompleteForm(IDialogContext context, IAwaitable<RequestFormBase> result)
        {
            try
            {
                var requestForm = await result;
                await requestForm.CompleteForm(context);
            }
            catch (Exception exception)
            {
                HandleFormException(context, exception);
                context.Done(string.Empty);
            }
        }

        public static async void HandleFormException(IDialogContext context, Exception exception)
        {
            if (exception is FormCanceledException)
            {
                string reply;
                if (exception.InnerException == null)
                {
                    // User cancelled the form
                    reply = "It was great talking to you! I hope I was helpful.";
                    PrivateTracer.Tracer.TraceInformation("User decided to quit the form");
                }
                else
                {
                    // Exception was thrown
                    reply = Prompts.FormExceptionMessage;
                    DiagnosticsTrace.TraceException($"Failure to complete form. Exception:", exception, PrivateTracer.Tracer);
                }
                await context.PostAsync(reply);
            }
            else
            {
                DiagnosticsTrace.TraceException($"Failure to open request ticket. Exception:", exception, PrivateTracer.Tracer);
                await context.PostAsync(Prompts.FormExceptionMessage);
            }
        }

        #region Tracer
        private class PrivateTracer : TraceSourceBase<PrivateTracer>
        {
            /// <summary>
            /// The string that identifies this trace source
            /// </summary>
            private const string c_identifierString = "KusBot.FormDialog";

            /// <summary>
            /// Implements <see cref="TraceSourceBase{T}.Id"/>
            /// </summary>
            public override String Id
            {
                get { return c_identifierString; }
            }

            /// <summary>
            /// Implements <see cref="TraceSourceBase{T}.DefaultVerbosity"/>
            /// </summary>
            public override TraceVerbosity DefaultVerbosity
            {
                get { return TraceVerbosity.Verbose; }
            }
        }
        #endregion
    }
}