MVC4 - NullReferenceException being thrown in view when trying to use a list

On my home (index) page I have two partials, one renders a search form and the other results from the search:

<div class="row-fluid well">
    <div class="span6">
        @Html.Partial("~/Views/Search/_BasicPropertySearchPartial.cshtml")
    </div>
    <div class="span6" id="basic-property-search-results">
        @Html.Partial("~/Views/Search/_BasicPropertySearchResultsPartial.cshtml")
    </div>
</div>

In my SearchController a GET action returns the search form:

[HttpGet]
public ActionResult BasicPropertySearch()
{
    return PartialView("_BasicPropertySearchPartial");
}

And a POST action gets user input from the form and returns results based on a query:

[HttpPost]
public ActionResult BasicPropertySearch(BasicPropertySearchViewModel viewModel)
{
    var predicate = PredicateBuilder.True<ResidentialProperty>();
    if (ModelState.IsValid)
    {
        using(var db = new LetLordContext())
        {
            predicate = predicate.And(x => x.HasBackGarden);
            //...
            var results = db.ResidentialProperty.AsExpandable().Where(predicate).ToList();

            GenericSearchResultsViewModel<ResidentialProperty> gsvm = 
                    new GenericSearchResultsViewModel<ResidentialProperty> { SearchResults = results };

            return PartialView("_BasicPropertySearchResultsPartial", gsvm);
        }            
    }
    ModelState.AddModelError("", "Something went wrong...");
    return View("_BasicPropertySearchPartial");
}

I've created a generic view model because search results may be lists of different types:

public class GenericSearchResultsViewModel<T>
{
    public List<T> SearchResults { get; set; }

    public GenericSearchResultsViewModel()
    {
        this.SearchResults = new List<T>();
    }
}

The POST action returns the following view:

@model LetLord.ViewModels.GenericSearchResultsViewModel<LetLord.Models.ResidentialProperty>

@if (Model.SearchResults == null) // NullReferenceException here!
{
    <p>No results in list...</p>
}
else
{
    foreach (var result in Model.SearchResults)
    {
    <div>
        @result.Address.Line1
    </div>
    }
}

I've put breakpoints on the GET and POST actions and the exception is being thrown before either or hit.

Is this problem being caused because index.cshtml is being rendered before it has a chance to do the GET/POST in the SearchController?

If so, does this mean it's a routing problem?

Finally, I thought newing SearchResults in the constructor would overcome NullReferenceExceptions?

Feedback appreciated.