private void SalesUi_Load(object sender, EventArgs e)
{
categoryComboBox.DataSource = _salesManager.CategoryCombo();
categoryComboBox.Text = "-Select-";
productComboBox.DataSource = _salesManager.ProductCombo();
productComboBox.Text = "-Select-";
customerComboBox.DataSource = _salesManager.CustomerCombo();
customerComboBox.Text = "-Select-";
}