my update panel not update ..?

this is my grid view selectedindexchanged..

my drop down list control not working ....occur exception


protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
{

Txtstudentname.Text = GridView1.SelectedRow.Cells[1].Text;
DropDownListcollegename.Text = GridView1.SelectedRow.Cells[2].Text;
DropDownListdegree.Text = GridView1.SelectedRow.Cells[3].Text;
Txtmobilenumber.Text = GridView1.SelectedRow.Cells[4].Text;
DropDownListpurpose.Text = GridView1.SelectedRow.Cells[5].Text;
DropDownListdistrict.Text = GridView1.SelectedRow.Cells[6].Text;
Txtremindingdate.Text = GridView1.SelectedRow.Cells[7].Text;
}