Get Drop Down Value

Question

Hello Every One i have use this code for get value in drop down list .But i have face issue.This is my code

public IEnumerable<SelectListItem> GetPlanList()
{
var PlanList = _context.PlanMaster.OrderByDescending(x => x.PlanId)
.Select(x => new SelectListItem()
{
Text = x.PlanName,
Value = x.PlanId.ToString()
}).ToList(); --- it's me c# code
return PlanList;

This is my script

$(document).ready(function () {
debugger;
$("#DropDown").change(function () {
$.get("/AddCoupan/GetPlanList", { PlanId: $("#DropDown").val() }, function (data) {
$.each(data, function (index, row) {
$("#DropDown").append("<option value='" + row.planId + "'>" + row.planName + "</option>")
});
});
})
});
</script>

@Html.DropDownListFor(x=>x.PlanId, Model.PlanMaster,"--Select Plan--",new { @class = "form-control", @id = "DropDown", @required = "required" })--But Here I have Face Issue object reference not set to an instance of an object

 

It's view model class

public int PlanId { get; set; }
public IEnumerable<SelectListItem> PlanMaster { get; set; }
public string PlanName { get; set; }