Value cannot be null. Parameter name: items (in Dropdown List) ASP.NET MVC5

You cannot bind a <select> element to a complex object (which is what Role is) and when you submit the form, ModelState is invalid (your trying to bind the selected Name property of RolesList to typeof IdentityRole). You then return the view, but have not repopulated the RolesList property, so its null (hence the error).

View models should not contain data models, and you view model should be

public class RegisterViewModel
{
    ....
    [Required(ErrorMessage = "Please select a role")]
    public string Role { get; set; }
    public IEnumerable<SelectListItem> RoleList { get; set; }
}

and in the GET method

var roles = _context.Roles.Select(r => r.Name);
var viewModel = new RegisterViewModel
{
    RolesList = new SelectList(roles)
};
return View(viewModel);

and in the view

@Html.LabelFor(m => m.Role, new { @class = "col-md-2 control-label" })
<div class="col-md-10">
    @Html.DropDownListFor(m => m.Role, Model.RolesList, "Select Role", new { @class = "form-control" })
</div>

this will solve the invalid ModelState issue relating to Role, but if you do need to return the view because of other ModelState issues, then you must first repopulate the collection before returning the view

if (!ModelState.IsValid)
{
    var roles = _context.Roles.Select(r => r.Name);
    model.RolesList = new SelectList(roles);
    return View(model);
}
.... // save and redirect