Add null check for WaitInfo

[CoffeeFlux]

Fixes #49583

[jkotas]

I think the problem is caused by Thread constructors that throw exceptions before being fully initialized. Consider:

        public Thread(ThreadStart start)
        {
            if (start == null)
            {
                // If we take this path, we will have Thread object with null WaitInfo
                throw new ArgumentNullException(nameof(start));
            }

            _startHelper = new StartHelper(start);

            Initialize();
        }

[CoffeeFlux]

If the issue is the constructor, I don't understand why it would be flaky. I guess it's because most of the time the finalizer won't run before the runtime as a whole gets shut down? This should at least be easy to test.

[CoffeeFlux]

Looks like you're right:

using System;
using System.Threading;

namespace HelloWorld
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            try {
                new Thread((ThreadStart)null);
            }
            catch {}
            GC.Collect();
            GC.WaitForPendingFinalizers();
            Console.WriteLine("done");
        }
    }
}

I didn't even consider that finalizers are run in C# if the constructor throws... too used to C++. I guess in that case the null check is fine, and I'll move it as suggested. Thanks!

[CoffeeFlux]

Failure is #49569