NullReferenceException ASP.NET MVC 5

I have a web application with this controller:

    public class ServiceRequestController : Controller
    {

    [Authorize(Roles = "Customer")]
    public ActionResult Create()
    {
        return View();
    }

    [Authorize(Roles = "Customer")]
    public ActionResult CreateNewUserAccount()
    {
        return View();
    }

    [Authorize(Roles = "Customer")]
    [HttpPost]
    public ActionResult CreateNewUserAccount(ServiceRequest serviceRequest)
    {
        if (ModelState.IsValid)
        {
            serviceRequest.Log.Id = User.Identity.GetUserId().ToString();
            serviceRequest.Log.DateTimeLogged = System.DateTime.Now;
            serviceRequest.LogID = db.Logs.Max(item => item.LogID);
            serviceRequest.EstimatedResolveDate serviceRequest.CalculateEstimatedResolveDate();
            db.ServiceRequests.Add(serviceRequest);
            db.SaveChanges();
            return RedirectToAction("AllServiceRequests", "Log");
        }
        return View(serviceRequest);
    }

The serviceRequest.Log.Id = User.Identity.GetUserId().ToString(); (And any preceding line if this is commented out) throws a null reference exception. I presume the serviceRequest is somehow null?

The ActionLink which requests the CreateNewUserAccount() page is:

@Html.ActionLink("New User Account", "CreateNewUserAccount", "ServiceRequest")

I'm not sure how to resolve this exception?

The model is:

public partial class ServiceRequest
{
    public int ServiceRequestID { get; set; }
    public Nullable<int> LogID { get; set; }
    public string RequestType { get; set; }
    [DisplayName("Additional Information")]
    [Required]
    [StringLength(200)]
    public string AdditionalInformation { get; set; }
    public DateTime EstimatedResolveDate { get; set; }

    [Required]
    [DisplayName("Delivery Date")]
    public DateTime DeliveryDate { get; set; }

    public virtual Log Log { get; set; }


    public DateTime CalculateEstimatedResolveDate()
    {
        return System.DateTime.Now.AddDays(3);
    }

}

View code:

@model OfficiumWebApp.Models.ServiceRequest

@{
ViewBag.Title = "New User Account";
}
@using(Html.BeginForm("CreateNewUserAccount", "ServiceRequest", FormMethod.Post))
{
@Html.AntiForgeryToken()
<div class="form-horizontal">
    @Html.ValidationSummary(true)

    <div class="form-group">
        @Html.LabelFor(model => model.RequestType, new { @class = "control-label col-md-2" })
        <div class="col-md-3">
            <div class="editor-field">
                @Html.TextBoxFor(model => model.RequestType, new { @Value = ViewBag.Title, @readonly = "readonly" })
                @Html.ValidationMessageFor(model => model.RequestType)
            </div>
        </div>
    </div>

    <div class="form-group">
        @Html.Label("Name of Account Holder", new { @class = "control-label col-md-2" })
        <div class="col-md-3">
            <div class="editor-field">
                @Html.TextBox("AccountName")
                @Html.ValidationMessageFor(model => model.RequestType)
            </div>
        </div>
    </div>

   <div class="form-group">
    @Html.Label("Department", new { @class = "control-label col-md-2" })
    <div class="col-md-3">
        <div class="editor-field">
            @Html.TextBox("Department")
            @Html.ValidationMessageFor(model => model.RequestType)
        </div>
    </div>
</div>

<div class="form-group">
        @Html.Label("Location", new { @class = "control-label col-md-2" })
        <div class="col-md-3">
            <div class="editor-field">
                @Html.TextBox("Location", null, new { id = "Location" }))
                @Html.ValidationMessageFor(model => model.RequestType)
            </div>
        </div>
    </div>


    <div class="form-group">
        @Html.LabelFor(model => model.AdditionalInformation, new { @class = "control-label col-md-2" })
        <div class="tags">
            <div class="col-md-10">
                @Html.TextAreaFor(model => model.AdditionalInformation)
                @Html.ValidationMessageFor(model => model.AdditionalInformation)
            </div>
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.DeliveryDate, new { @id = "VisitDateLabel", @class = "control-label col-md-2" })
        <div class="col-md-3">
            <div class="editor-field">
                @Html.JQueryUI().DatepickerFor(model => model.DeliveryDate).Inline(false)
                @Html.ValidationMessageFor(model => model.DeliveryDate)
            </div>
        </div>
    </div>
        <div class="form-group">
        <div class="col-md-offset-2 col-md-10">
            <button type="submit" class="btn btn-default"><span class="glyphicon glyphicon-floppy-save"></span></button>
        </div>
    </div>
</div>
}