private void btnAdd_Click(object sender, EventArgs e)
        {
            // Opens AddCustomer form and ensures no record is currently selected
            GlobalVariables.selectedCustomerID = 0;
            frmAddCustomer editForm = new frmAddCustomer();

            editForm.ShowDialog();
            lvCustomers.Items.Clear();
            DisplayCustomers();
        }
        private void btnUpdate_Click(object sender, EventArgs e)
        {
            // Checks user has selected a customer
            if (lvCustomers.SelectedItems.Count == 0)
            {
                MessageBox.Show("Please select a Customer to update.");
                return;
            }
            // Stores CustomerID to be updated
            GlobalVariables.selectedCustomerID = int.Parse(lvCustomers.SelectedItems[0].Text);

            // Opens AddCustomer form
            frmAddCustomer editForm = new frmAddCustomer();

            editForm.ShowDialog();
            lvCustomers.Items.Clear();
            DisplayCustomers();
        }