DropDownList in DetailsView not working

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ASP.NET

Hi,
I am using DetailsView in my web application. There is DropDownList in view in Insert and Edit mode. now when i open DetailsView in Insert mode then i get dropdown control without any error.

But, when i open DetailsView in edit mode i am not getting DropDownList control

I am using following code

if (dvCategoryMaster.CurrentMode == DetailsViewMode.Edit)
{
     DropDownList ddlParentCategory = (DropDownList)dvCategoryMaster.FindControl("ddl");
     ddlParentCategory.Items.FindByValue(categoryInstance.Category_Parent_ID.ToString()).Selected = true;
}

Here i am getting ddlParentCategory = null

So kindly help me, how to solve this error?