DropDownList causes error: Object reference not set to an instance of an object

I keep getting this error and have no idea why. I googled and scanned the asp.net site for two days now so I need some help. The error is:

Object reference not set to an instance of an object

Here's my code:

DropDownList DropDownList1 =(DropDownList)ListView1.InsertItem.FindControl("DropDownList1");

string highToLow = DropDownList1.SelectedValue;
string lowToHigh = DropDownList1.SelectedValue;

if (highToLow == "1")
{
    var exmapleFilter = from users in testEntities.users 
                        orderby users.id descending
                        select users;

    ListView1.DataBind();
}

I have the value of the drop down set to 1 for high and 2 for low, and on selected index changed I want to run the ADO.net Entity Framework code to return a sorted list of data.

I'm currently using a linq data source and a list view to show what's in my database.

Thanks.

EDIT:

Here is the stack trace

   System.NullReferenceException was unhandled by user code
   Message=Object reference not set to an instance of an object.
   Source=App_Web_s0ked5y3

   StackTrace:
   at Default.DropDownList1_SelectedIndexChanged(Object sender, EventArgs e)
   in Default:line 120

   at System.Web.UI.WebControls.ListControl.OnSelectedIndexChanged(EventArgs e)

   at System.Web.UI.WebControls.DropDownList.RaisePostDataChangedEvent()

   at System.Web.UI.WebControls.DropDownList.System.Web.UI.IPostBackDataHandler.
   RaisePostDataChangedEvent()

   at System.Web.UI.Page.RaiseChangedEvents()

   at System.Web.UI.Page.ProcessRequestMain(Boolean 
   includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   InnerException: