How to throw exception if it doesn't get in if-statement?

I want to explain the mistake to user by using exception.

An error occurs because the column we want to delete during the deletion cannot be empty.

private void button2_Click(object sender, EventArgs e)
{
    if (MessageBox.Show("Delete this Row !!!", 
                        "YES", 
                         MessageBoxButtons.YesNo, 
                         MessageBoxIcon.Question) == DialogResult.Yes)
    {
        if (dataGridView1.CurrentRow != null)
        {
            int id = (int)dataGridView1.CurrentRow.Cells["WAREHOUSE_ID"].Value;

            Alfa.Entities.Models.WAREHOUSE w = managerWAREHOUSE
              .Find(x => x.WAREHOUSE_ID == id);

            int sonuc = managerWAREHOUSE.Delete(w);

            if (sonuc > 0)
            {
                MessageBox.Show("Record deleted");
                Listele();
            }
            else
            {
                MessageBox.Show("Unable to delete record");
            }
        }
    }
}

Actually it has to enter to the else statement. But before if statement it gives error.