DataGridViewComboBoxColumn Stackoverflow issue

private void drvRtn_EditingControlShowing(object sender,DataGridViewEditingControlShowingEventArgs e) 
{ 
     if (drvRtn.CurrentCell.ColumnIndex == 21) 
    { 
       ComboBox comboBox = e.Control as ComboBox; comboBox.SelectedIndexChanged += new EventHandler(comboBox_SelectedIndexChanged); 
     } 
} 

void comboBox_SelectedIndexChanged(object sender, EventArgs e) 
{ 
     if (drvRtn.CurrentCell.ColumnIndex == 21 ) 
     { 
         int selectedIndex = ((ComboBox)sender).SelectedIndex; if (selectedIndex == 0) 
         { 
             DateTime time1 = DateTime.Now; 
             time1 = DateTime.Now; 
             drvRtn[22, 0].Value = "CONTACTABLE-NO ADD CHANGE" ;
              drvRtn[21, 0].Value = "CONTACTABLE-NO ADD CHANGE" ;
              string format = "HH:mm:ss"; 
               drvRtn[19, 0].Value = time1.ToString(format); 
              drvRtn[18, 0].Value = time1.ToString(); 
              drvRtn[23, 0].Value = time1.ToString(); 
        } 
     }
}


private void drvRtn_CellClick_1(object sender, DataGridViewCellEventArgs e) 
{ 
    columnIndex = drvRtn.CurrentCell.ColumnIndex; if (columnIndex == 17) 
    { 
        DataGridViewComboBoxColumn colRtnRsn = new DataGridViewComboBoxColumn(); 
        colRtnRsn.Items.AddRange("NONE", "SELF", "HUSBAND", "MOTHER", "FATHER", "SISTER"); 
        //colRtnRsn.DataPropertyName = "spoke_to1"; 
        //  colRtnRsn.ValueMember = "spoke_to1"; 
        //  colRtnRsn.DisplayMember = "spoke_to1"; 
        //  colRtnRsn.HeaderText = "spoke_to1"; 
         // colRtnRsn.Width = 120; 
       //colRtnRsn.AutoSizeMode = DataGridViewAutoSizeColumnMode.NotSet; 
       if (Ctr2 == 0) 
       { this.drvRtn.Columns.Remove("spoke_to1"); 
         this.drvRtn.Columns.Insert(17, colRtnRsn); 
         Ctr2 = Ctr2 + 1; 
       }
    } 
}