DataGridTableStyle: Object Reference not set to an instance of an obj

[whloo]

public void test(DataGrid _dataGrid, string _mappingName)
{
DataGridTableStyle oldTS = _dataGrid.TableStyles[_mappingName];
...

}

DataTable xDT = getDataFromDB();
ResultGrid.DataSource = xDT;
test(ResultGrid, "Table");

Hi,

the code above gave me some funny problem.
I able to get all data from db and put it into xDT and bind it to the ResultGrid.
then i try to do some grid manipulation using the test function, but seems like i encounter some problem when i call:

DataGridTableStyle oldTS = _dataGrid.TableStyles[_mappingName];

anyone know what's wrong with this? have been trying to find the root of the problem for few days
Thanks!