Null reference exception creates when datagrid rows are not visible within datagrid in C# WPF

Question

In C# WPF, Null reference exception creates when datagrid rows are not visible within grid (It occurs when there are rows which can be seen by scrolling down the datagrid) how can I avoid that? I am new to WPF.

My Code:

foreach (DataRowView dr in datagrid.ItemsSource)
        {
            DateTime date2 = Convert.ToDateTime(dr["date2"]);
            DateTime currentdate = DateTime.Now;
            DateTime upcomingdate = DateTime.Now.AddDays(+5);

            if (date2 < currentdate)
            {
                var Srow = datagrid.ItemContainerGenerator.ContainerFromItem(dr) as DataGridRow;
                Srow.Foreground = Brushes.Red;
            }
            else if (date2 < upcomingdate)
            {
                var Srow1 = datagrid.ItemContainerGenerator.ContainerFromItem(dr) as DataGridRow;
                Srow1.Foreground = Brushes.Green;
            }

        }