Test_StartEventMonitoring.cs

using Microsoft.VisualStudio.TestTools.UnitTesting;
using System;
using System.Collections.Generic;
using System.Threading.Tasks;
using OpenQA.Selenium;
using OpenQA.Selenium.Chrome;
using OpenQA.Selenium.Support.Events;
using OpenQA.Selenium.Support.UI;

namespace UnitTestProject1
{
    [TestClass]
    public class UnitTest1
    {
        private IWebDriver driver;

        [TestInitialize]
        public void TestInit()
        {
            var options = new ChromeOptions();
            options.AddArgument("enable-automation");
            options.AddArgument("disable-infobars");
            options.AddArgument("disable-notifications");
            options.AddArgument("start-maximized");
            options.AddUserProfilePreference("profile.default_content_settings.popups", 0);
            options.BinaryLocation = @"C:\Program Files\Chromium\Application\chrome.exe";
            
            var chromeService = ChromeDriverService.CreateDefaultService();
            var chromeDriver = new ChromeDriver(chromeService, options, TimeSpan.FromSeconds(60));
            driver = new EventFiringWebDriver(chromeDriver);
        }

        [TestMethod]
        public async Task TestMethod1()
        {
            List<DomMutationData> attributeValueChanges = new List<DomMutationData>();
            DefaultWait<List<DomMutationData>> wait = new DefaultWait<List<DomMutationData>>(attributeValueChanges);
            wait.Timeout = TimeSpan.FromSeconds(3);

            IJavaScriptEngine monitor = new JavaScriptEngine(driver);
            monitor.DomMutated += (sender, e) =>
            {
                attributeValueChanges.Add(e.AttributeData);
            };
            await monitor.StartEventMonitoring();

            driver.Navigate().GoToUrl("http://www.google.com");
            IWebElement span = driver.FindElement(By.CssSelector("span"));

            await monitor.EnableDomMutationMonitoring();
            ((IJavaScriptExecutor)driver).ExecuteScript("arguments[0].setAttribute('cheese', 'gouda');", span);

            wait.Until((list) => list.Count > 0);
            Console.WriteLine("Found {0} DOM mutation events", attributeValueChanges.Count);
            foreach (var record in attributeValueChanges)
            {
                Console.WriteLine("Attribute name: {0}", record.AttributeName);
                Console.WriteLine("Attribute value: {0}", record.AttributeValue);
            }

            await monitor.DisableDomMutationMonitoring();
        }

        [TestCleanup]
        public void Cleanup() => driver.Dispose();
    }
}

fails with:

Message: 
Test method UnitTestProject1.UnitTest1.TestMethod1 threw exception: 
System.NullReferenceException: Object reference not set to an instance of an object.

  Stack Trace: 
<.ctor>b__0()
Lazy`1.CreateValue()
Lazy`1.LazyInitValue()
Lazy`1.get_Value()
<StartEventMonitoring>d__24.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
TaskAwaiter.ThrowForNonSuccess(Task task)
TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
TaskAwaiter.GetResult()
<TestMethod1>d__2.MoveNext() line 45
--- End of stack trace from previous location where exception was thrown ---
TaskAwaiter.ThrowForNonSuccess(Task task)
TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
ThreadOperations.ExecuteWithAbortSafety(Action action)