The aspx code and codebehind does not match. You have pasted the wrong code.
Morever above code will not work.You
should not use the Databind() logic in PreRender.
Use the Databind() in Page_Load event and in the
if(!Postback)
{
}
code block.
In the GridView1_RowCommand, you should findControl and bind the DropdownList.