ASP.NET MVC ArgumentNullException: Value cannot be null. (Parameter 'source')

Question

I have an asp.net mvc application. I want to perform and update operation.I want to show you my code before explain my problem sorry I am not fluent in english thats why I prefer to show my code first.Here I have an index page that shows all items in database with a table

 @model IEnumerable<InOut.Models.Item>
@if (Model.Count() > 0)
{
    <table class="table table-bordered table-striped" style="width: 100%; text-align: center">
        <thead>
        <tr>
            <th>Item Name</th>
            <th>Borrower</th>
            <th>Lender</th>
            <th >Edit</th>
        </tr>
        </thead>
        <tbody>
        @foreach (var item in Model)
        {
            <tr>
                <td style="width: 25%">@item.borrower</td>
                <td style="width: 25%">@item.Lender</td>
                <td style="width: 25%">@item.ItemName</td>
                <td style="width: 13%">
                    <a asp-controller="Item" asp-action="Delete" asp-route-id="@item.id"  onclick="showAlert()"class="btn btn-danger" id="btn1" style="text-decoration: none; color: white">delete</a>
                    <a asp-controller="Item" asp-action="Update" asp-route-id="@item.id"  class="btn btn-primary" id="btn1" style="text-decoration: none; color: white">Update</a>
                </td>
            </tr>
        }
        </tbody>
    </table>
}
else
{
    <p> No Items created </p>
 }

Here is the index action for that view

  public IActionResult Index()
{
    IEnumerable<Item> items = _dbContext.Items;
    return View(items);
}

In the foreach loop I have a button to update the object on that row it has an action named "Update" in controller class.here is the code to get the item on that row.

//Get Item
public IActionResult Update(int? id)
{
    var item = _dbContext.Items.Single(o => o.id == id);
    return View(item);
}

this action sends the selected item properties to the update view.here is the code of view

<form method="post" asp-action="Create">
<input asp-for="id" hidden/>
<div class="form-group">
    <label >Item Name </label>
    <input type="text" class="form-control" aria-describedby="emailHelp" asp-for="ItemName">
</div>
<div class="form-group">
    <label >Borrower</label>
    <input type="text" class="form-control" asp-for="borrower">
</div>
<div class="form-group">
    <label >Lender</label>
    <input type="text" class="form-control" asp-for="Lender">
</div>

<button asp-controller="Item" asp-action="Update" type="submit" class="btn btn-primary" style="margin-top: 10px">Submit</button>

submit button sends properties to the overloaded action update with the parameter of item. after update operation I want it to show me the index page. here is the overloaded update action

  // Update Item 
[HttpPost]
[ValidateAntiForgeryToken]
public IActionResult Update(Item item)
{
    _dbContext.Update(item);
    _dbContext.SaveChanges();
    return View("Index");
}

Problem starts here. When I run application after submitting changes I want it to show me the index action update operation working well.but when it try to show me the index page it gives me this error.

ArgumentNullException: Value cannot be null. (Parameter 'source') and it refers to the if statement in the index page it

now if I use redirectToAction return instead of View like this...

 [HttpPost]
[ValidateAntiForgeryToken]
public IActionResult Update(Item item)
{
    _dbContext.Update(item);
    _dbContext.SaveChanges();
    return RedirectToAction("Index");
}

It works as I wanted. I want to know what is the problem with the first approach and is there any difference between those 2 return types?