private void Save_Customer_Details(object sender, RoutedEventArgs e)
        {
            if (!(String.IsNullOrEmpty(customerName.Text) || String.IsNullOrEmpty(customerPhone.Text))) // If customer name and phone are not empty
            {
                customerDetails.IsEnabled = false; // Disable the customer details groupbox
                Customer newCustomer = new Customer(customerName.Text, customerPhone.Text);
                if (newCustomer.Check())
                {
                    newCustomer.SaveCustomer();
                    customerPhone.ItemsSource = Customer.Phones;
                }
                vehiclePrice.Focus(); // Set focus
            }
            else
            {
                MessageBox.Show("Customer details must be filled in before saving"); // Prompt user to enter data

                if (String.IsNullOrEmpty(customerName.Text))
                {
                    customerName.Focus(); // Set focus to Customer Name
                }
                else
                {
                    customerPhone.Focus(); // Set focus to Customer Phone
                }
            }
        }