this error is coming may be because dataGridEksp.SelectedRows is coming null...
so following may help you,
instead of using ..
txt_rrugaEks.Text = dataGridEksp.Rows[i].Cells[0].Value.ToString();
use following...
txt_rrugaEks.Text = dataGridEksp.Rows[e.RowIndex].Cells[0].Value.ToString();
for rest continue like above
OR u can use...
i = e.RowIndex;