Срабатывает System.NullRaferenceException при попытке изменить данные в Datagridview.ComboboxColumn

private void dgrv1_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
{
   var cb = e.Control as DataGridViewComboBoxEditingControl;
   cb.SelectedIndexChanged -= this.cb_SelectedIndexChanged;
   cb.SelectedIndexChanged += this.cb_SelectedIndexChanged;
   cb.DropDown -= this.cb_DropDown;
   cb.DropDown += this.cb_DropDown;
   cb.DropDownClosed -= this.cb_DropDownClosed;
   cb.DropDownClosed += this.cb_DropDownClosed;
}
 
private void cb_DropDown (object sender, EventArgs e)
{ var cb = sender as DataGridViewComboBoxEditingControl;
   string text = cb.text; }
private void cb_SelectedIndexChanged(object sender, EventArgs e)
{ var cb = sender as DataGridViewComboBoxEditingControl;
   int ind = cb.SelectedIndex; 
   dgrv1.CommitEdit(DataGridViewDataErrorContexts.Commit);
   dgrv1.EndEdit;}
private void cb_DropDownClosed(object sender, EventArgs e)
{ var cb = sender as DataGridViewComboBoxEditingControl;
   dgrv1.EndEdit; }

private void cb_DropDown (object sender, EventArgs e)
{ 
 if(sender is DataGridViewComboBoxEditingControl cb)
   {
      string text = cb.text; 
   }
}