private void billingToolStripMenuItem_Click(object sender, EventArgs e)
{
Customer_Billing bill = new Customer_Billing();
bill.instance.MdiParent = this;
bill.instance.Show();
bill.instance.WindowState = FormWindowState.Maximized;
}
private void Customer_Billing_FormClosed(object sender, FormClosedEventArgs e)
{
Customer_Billing._instance = null;
}