How do I update a record with a foreign key?

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ASP.NET

entity-framework

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I have a table with a foreign key to another table.

I was able to create the MVC for the Create action but I can't seem to figure out the Update action.

I keep getting an error: An unhandled exception occurred while processing the request.

ArgumentNullException: Value cannot be null.
Parameter name: source

VM:

[Required]
[Display(Name = "Name")]
public string Name { get; set; }

[Required]
[Display(Name = "Description")]
public string Description { get; set; }

[Required]
[Display(Name = "Code before element including opening tag")]
public string Before { get; set; }

[Required]
[Display(Name = "Code after element including closing tag")]
public string After { get; set; }

public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
{
    if (Before == null || After == null)
        yield return new ValidationResult("Custom HTML Before and Custom HTML After must be specified");
}

[Required]
[Display(Name = "HTML Element")]
public int HTMLElement { get; set; }

public ICollection<HTMLElement> HTMLElements { get; set; }

Controller:

[HttpGet]
public async Task<IActionResult> UpdateCustomizedHTMLElement(int Id)
{
    var eElement = await db.CustomizedElements
.Include(e => e.HTMLElement)
.ThenInclude(e => e.Element)
.SingleOrDefaultAsync(w => w.Id == Id);
    return View(eElement);
}

[HttpPost]
[ValidateAntiForgeryToken]
public IActionResult UpdateCustomizedHTMLElement(HTMLElement updateHTMLElement)
{
    return View(updateHTMLElement);
}

View:

<div class="row justify-content-md-center">
    <div class="col-sm-10 col-md-7 col-lg-6">
        <h2>Update Customized HTML Element Type</h2>
        <form method="post" asp-controller="Templates" asp-action="UpdateCustomizedHTMLElement">
            <div class="form-group">
                <label asp-for="Name"></label>:
                <input class="form-control form-control-sm" type="text" asp-for="Name" />
                <span class="text-danger" asp-validation-for="Name"></span>
            </div>
            <div class="form-group">
                <label asp-for="Before"></label>:
                <input class="form-control form-control-sm" type="text" asp-for="Before" />
                <span class="text-danger" asp-validation-for="Before"></span>
            </div>
            <div class="form-group">
                <label asp-for="HTMLElement"></label>:
                @Html.DropDownListFor(m => m.HTMLElement, new SelectList(Model.HTMLElement.Element, "Id", "Element"), "", new { @class = "form-control" })
                <span class="text-danger" asp-validation-for="HTMLElement"></span>
            </div>
            <div class="form-group">
                <label asp-for="After"></label>:
                <input class="form-control form-control-sm" type="text" asp-for="After" />
                <span class="text-danger" asp-validation-for="After"></span>
            </div>
            <div>
                <button class="btn btn-primary" type="submit" value="Submit">Save</button>
                <button class="btn btn-secondary" type="button" value="Cancel" onclick="location.href='@Url.Action("HTMLElements", "Templates")'">Cancel</button>
            </div>
        </form>
    </div>
</div>

What I have tried:

I have tried to Google "how to update a record with a foreign key property .net core" but haven't found anything that works.