Search datagrid if value is null or no data C#

System.NullReferenceException: 'Object reference not set to an instance of an object.'

System.Windows.Forms.DataGridViewCell.Value.get returned null.

private void ListDataGridView_CellFormatting(object sender, DataGridViewCellFormattingEventArgs e)
       {
           foreach (DataGridViewRow row in ListDataGridView.Rows)
           {
               if (String.IsNullOrWhiteSpace(row.Cells["R.b."].Value.ToString()))
               {
                   row.DefaultCellStyle.ForeColor = System.Drawing.ColorTranslator.FromHtml("#51B3E8");
               }
               else
               {
                   row.DefaultCellStyle.ForeColor = System.Drawing.ColorTranslator.FromHtml("#09ECA6");
               }
           }
       }