Why is my Castle Windsor controller factory's GetControllerInstance() being called with a null value?

Question

I am using Castle Windsor to manage controller instances (among other things). My controller factory looks like this:

public class WindsorControllerFactory : DefaultControllerFactory
    {
        private WindsorContainer _container;

        public WindsorControllerFactory()
        {
            _container = new WindsorContainer(new XmlInterpreter());

            var controllerTypes = from t in Assembly.GetExecutingAssembly().GetTypes()
                                  where typeof(Controller).IsAssignableFrom(t)
                                  select t;

            foreach (Type t in controllerTypes)
            {
                _container.AddComponentLifeStyle(t.FullName, t, LifestyleType.Transient);
            }
        }

        protected override IController GetControllerInstance(Type controllerType)
        {
            return (IController)_container.Resolve(controllerType); // ArgumentNullException is thrown here
        }

When I start up my ASP.Net MVC application and try to go to "/" (or another path), I get an ArgumentNullException. I put a break point on entry of the GetControllerInstance and found that it's called once with my HomeController, then a second time with null (which is when the exception is thrown). Why is it being called again?

Should I change the method to something like this:

protected override IController GetControllerInstance(Type controllerType)
{
    if (controllerType == null)
        return null;

    return (IController)_container.Resolve(controllerType);
}

Answer 1

It turns out that the second request was the MVC framework trying to find a script I included in the Site.Master. The path did not exist, so I guess it tried to resolve a controller (that matched /Scripts/sitescripts.js). I changed the method to this:

protected override IController GetControllerInstance(Type controllerType)
{
    if (controllerType != null)
    {
       return (IController)_container.Resolve(controllerType);
    }
    else
    {
       return base.GetControllerInstance(controllerType);
    }
}

And an exception with an understandable message was thrown.