based on what I see it must be that you activeForm has not be initialized and is null. Without more code can't be certain.

after looking a little more, it may be better to use the cellvaluechanged event... sample (note this was tested only in C# win32.forms, and the column ValueTypes are strings):

private void dataGrid_CellValueChanged(object sender, DataGridViewCellEventArgs e) {
/*check Changing cell is "not" in the "Total" Column,
*would keep calling cell changed event, when the "Total" cell value changes*/
if ((e.RowIndex >= 0) && e.ColumnIndex != DataGrid.Columns["Total"].Index) {
//r = row of the changed Cell.
int r = e.RowIndex;

      //get values from cells in the same row
      int Qty = 0;
      int.TryParse((string)DataGrid\["Quantity", r\].Value, out Qty);

      double Price = 0D;
      double.TryParse((string)DataGrid\["UnitPrice", r\].Value, out Price);

      //Calculate Total, Change cell value in "Total"
      DataGrid\["Total", r\].Value = Price \* Qty;
 }

}

Have you tried to update the values in the CellLeave event for your datagrid.