private void thongKeDoanhThuToolStripMenuItem_Click(object sender, EventArgs e)
{
frmThongKeDoanhThu frm = frmThongKeDoanhThu.Instance;
ShowTabages("Thống kê doanh thu", frm);
SetSelectedTab("Thống kê doanh thu");
}
protected override void OnClosed(EventArgs e)
{
base.OnClosed(e);
_instance = null;
}