ASP.Net GridView RowUpdating Error: Index was out of range. Must be non-negative and less than the size of the collection

Facing this issue when i am updating department in Employee Detail table using dropdownlist on event of Updating.

Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index

On Gridview_RowUpdating , on following line getting this erro

string NamDept =(GridView.Rows[e.RowIndex].FindControl("ddlDepartment") as DropDownList).SelectedItem.Value;

        SqlConnection myConn = default(SqlConnection);
        SqlCommand sqComm = default(SqlCommand);
 
        System.Data.DataSet ds = new System.Data.DataSet();
        System.Data.SqlClient.SqlDataAdapter SqlAdapter;
        protected void Page_Load(object sender, EventArgs e)
        {
            if (setConn())
            {
                PopulateDS();       // FILL DATASET WITH MASTER DATA.
                ShowEmpDetails();   // SHOW EMPLOYEE DETAILS IN THE GRIDVIEW.
            }
        }
        private bool setConn()
        {
            // SET DATABASE CONNECTION.
            try
            {
                myConn = new SqlConnection("Data Source=IT;Integrated Security=SSPI;Initial Catalog=AttendanceManagement");
                myConn.Open();
 
                sqComm = new SqlCommand();
                sqComm.Connection = myConn;
            }
            catch (Exception ex) { return false; }
            return true;
        }
        // CANCEL ROW EDITING.
        protected void GridView_RowCancelingEdit(object sender, GridViewCancelEditEventArgs e)
        {
            GridView.EditIndex = -1;
            ShowEmpDetails();
        } 
 
        protected void GridView_RowDataBound(object sender, GridViewRowEventArgs e)
        {
            if ((e.Row.RowState & DataControlRowState.Edit) > 0)
            {
                // BIND THE "DROPDOWNLIST" WITH THE DATASET FILLED WITH "QUALIFICATION" DETAILS.
                DropDownList ddldept = new DropDownList();
                ddldept = (DropDownList)e.Row.FindControl("ddlDepartment");
 
                if (ddldept != null)
                {
                    ddldept.DataSource = ds.Tables["Department"];
                    ddldept.DataTextField = ds.Tables["Department"].Columns["NamDept"].ColumnName.ToString();
                    ddldept.DataValueField = ds.Tables["Department"].Columns["IDdept"].ColumnName.ToString();
                    ddldept.DataBind();
 
                    // ASSIGN THE SELECTED ROW VALUE ("QUALIFICATION CODE") TO THE DROPDOWNLIST SELECTED VALUE.
                    ((DropDownList)e.Row.FindControl("ddlDepartment")).SelectedValue = DataBinder.Eval(e.Row.DataItem, "IDdept").ToString();
                }
            }
        }
 
        protected void GridView_RowEditing(object sender, GridViewEditEventArgs e)
        { 
            GridView.EditIndex = e.NewEditIndex;
            ShowEmpDetails(); 
        }
        // MASTER DATA IN A DATASET.
        private void PopulateDS()
        {
            ds.Clear();
            SqlAdapter = new System.Data.SqlClient.SqlDataAdapter("select NamDept,IDdept from Department", myConn);
            SqlAdapter.Fill(ds, "Department");
            SqlAdapter.Dispose();
        }
 
        private void ShowEmpDetails()
        {
            string sQuery = "SELECT EmployeeDetails.EmpID ,EmployeeDetails.EmpName, EmployeeDetails.EmpContact, Department.NamDept , SubDepartment.NamSubDept ,EmployeeDetails.Status,EmployeeDetails.JoinDate,EmployeeDetails.LeftDate,EmployeeDetails.Empcur,EmployeeDetails.Lentitled,EmployeeDetails.PL,EmployeeDetails.Address,EmployeeDetails.OTEntitled,EmployeeDetails.LTime,EmployeeDetails.OTRate,EmployeeDetails.Dhour,Designation.NamDesig FROM  EmployeeDetails INNER JOIN  Department ON Department.IDDept = EmployeeDetails.IDDept INNER JOIN SubDepartment ON SubDepartment.IDSubDept = EmployeeDetails.IDSubDept INNER JOIN Designation ON Designation.IDDesig = EmployeeDetails.IDDesig where empcur = 'join'";
 
            SqlDataReader sdrEmp = GetDataReader(sQuery);
            try
            {
                if (sdrEmp.HasRows)
                {
                    DataTable dt = new DataTable();
                    dt.Load(sdrEmp);
 
                    GridView.DataSource = dt;
                    GridView.DataBind();        // BIND DATABASE TABLE WITH THE GRIDVIEW.
                }
            }
            catch (Exception ex) { }
            finally
            {
                sdrEmp.Close();
                sdrEmp = null;
            }
        }
        private SqlDataReader GetDataReader(string sQuery)
        {
            SqlDataReader functionReturnValue = default(SqlDataReader);
            sqComm.CommandText = sQuery;
            sqComm.ExecuteNonQuery();
            functionReturnValue = sqComm.ExecuteReader();
            sqComm.Dispose();
            return functionReturnValue;
        }
 
        protected void GridView_RowUpdating(object sender, GridViewUpdateEventArgs e)
        {
            string NamDept = (GridView.Rows[e.RowIndex].FindControl("ddlDepartment") as DropDownList).SelectedItem.Value;
        }