private void salestatisticstBttn_Click(object sender, EventArgs e)
 {
     if (salesstatisticsform == null)
     {
         salesstatisticsform             = new salesstatisticsForm();
         salesstatisticsform.FormClosed += salesstatisticsform_FormClosed;
     }
     salesstatisticsform.Show(this);
     Hide();
 }
 void salesstatisticsform_FormClosed(object sender, FormClosedEventArgs e)
 {
     salesstatisticsform = null;
     Show();
 }