Microsoft Bot Framework unit tests not continuing dialogs - botframework

Question

I have a MSFT Bot Framework application for which I am trying to write unit tests. I have single call/response tests working fine, but anything that requires continuing a dialog does not work. The ActiveDialog property in the DialogContext that get's created on every turn is always null and the Stack property is always empty. I was sort of following this blog post example. What am I missing that allows the bot to maintain it's state between turns?
One-Time Setup
protected virtual void TestFixtureSetup()
{
var environmentName = "development";
var builder = new ConfigurationBuilder();
var configuration = builder.Build();
_connectedServices = new BotServices(_botConfig, configuration, environmentName);
_testAdapter = new TestAdapter();
_testAdapter.Use(new AutoSaveStateMiddleware());
}
Per-Test Setup
protected virtual void TestSetup()
{
var memStore = new MemoryStorage();
var userState = new UserState(memStore);
var conversationState = new ConversationState(memStore);
var dialogState = conversationState.CreateProperty<DialogState>("dialogState");
_dialogSet = new DialogSet(dialogState);
_dialogSet.Add(new MainDialog(_connectedServices, conversationState, userState, new TestTelemetryClient()));
_testFlow = new TestFlow(_testAdapter, async (turnContext, cancellationToken) =>
{
var dc = await _dialogSet.CreateContextAsync(turnContext, cancellationToken);
await dc.ContinueDialogAsync();
if (!turnContext.Responded)
{
await dc.BeginDialogAsync(nameof(MainDialog));
}
});
}
Test
public async Task MenuTestMethod(string subMenuOption, string verificationString)
{
var firstMenu = Responses.BuildFirstMenu(null, null);
var secondMenu = Responses.BuildSecondMenu(null, null);
await _testFlow
.Send("go to first menu")
.AssertReply((activity) =>
{
Assert.AreEqual(firstMenu.Attachments[0].Content, activity.AsMessageActivity().Attachments[0].Content);
})
.Send("go to second menu")
.AssertReply((activity) =>
{
Assert.AreEqual(secondMenu.Attachments[0].Content, activity.AsMessageActivity().Attachments[0].Content);
})
.Send(subMenuOption)
.AssertReply((activity) =>
{
Assert.IsTrue(activity.AsMessageActivity().Text.Contains(verificationString));
})
.StartTestAsync();
}