private void btnThem_Click(object sender, EventArgs e)
{
panelPhieuNhap.Enabled = true;
btnLuu.Enabled = true;
btnSua.Enabled = true;
btnThemPhieuNhap.Enabled = true;
txtMaSoPhieu.Text = qlkho.KiemTraTrung();
txtDienGiai.Text = string.Empty;
btnThemSP.Enabled = false;
btnXoaSP.Enabled = false;
cboNhanVien.Enabled = true;
cboKho.Enabled = true;
}