Object reference not set... error, when DataGrid MouseUp

<DataGrid x:Name="gridTable1">
    <DataGrid.ItemContainerStyle>
        <Style TargetType="DataGridRow">
            <EventSetter Event="MouseUp" Handler="dataGridRow_MouseUp"/>
        </Style>
    </DataGrid.ItemContainerStyle>
</DataGrid>

private void dataGridRow_MouseUp(object sender, MouseButtonEventArgs e)
   {
       DataGridRow dgr = sender as DataGridRow;
       DataRowView drv = dgr.DataContext as DataRowView; //This error: Object reference not set to an instance of an object

       string requestId = drv.Row["RequestID"].ToString();

       DataTable dt = new DataTable();
       dt = db.MySelect("select RequestID from Requests where RequestID='" + requestId + "'");
       gridTable2.ItemsSource = dt.DefaultView;
   }