MVC controller wants empty contructor but then Interface is not used and throws error
<p>I am getting this error:</p>

<p><code>An exception of type 'System.NullReferenceException' occurred in PubStuff.Intern.Web.Internal.dll but was not handled in user code
Additional information: Object reference not set to an instance of an object</code></p>

<pre><code>public class InternController : BaseController
{
    IInternService _internService;


    public InternController() { }

    public InternController(IInternService internService)
    {
        _internService = internService;
    }


    // GET: Intern
    public ActionResult Index()
    {
        object responseObject = null;


        responseObject = _internService.GetAllSkills();

        return View();
    }
}
</code></pre>

<ol>
<li>It complains If I do not have empty constructor</li>
<li>Once there IS an empty contructor then this line <code>responseObject = _internService.GetAllSkills();</code> throws the error.</li>
</ol>

<p>_internService is null</p>

<p><strong>How do I fix this?</strong>  <strong>What are the problem(s)?</strong></p>

<p><strong>Updates</strong>
I end up having a problem with <strong>StructureMap</strong> whether I add <strong>IInternUnitOfWork</strong> or NOT.</p>

<p>I added the <code>IInternService</code> to StructureMap and then doesn't help </p>

<p><strong>Error thrown</strong></p>

<pre><code>protected override object DoGetInstance(Type serviceType, string key)
    {
        if (string.IsNullOrEmpty(key))
        {
            return serviceType.IsAbstract || serviceType.IsInterface
                       ? this.Container.TryGetInstance(serviceType)
                       : this.Container.GetInstance(serviceType);
        }

        return this.Container.GetInstance(serviceType, key);
    }
</code></pre>

<p><code>"StructureMap Exception Code:  202\nNo Default Instance defined for PluginFamily PublicHealth.Intern.DataAccess.Contracts.IInternUnitOfWork, PublicHealth.Intern.DataAccess.Contracts, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"}</code></p>