Without onchange and default,how to get the invoice_number  in register payment wizard in saleorder in odoo12

In saleorder I have created a wizard Register payment..While clicking on register payment I want to get the invoice numbers based on active_ids and check if partner invoices address are different in selected active_ids otherwise raise warning...

Here is my code with onchange...

# -*- coding: utf-8 -*-

from odoo import models, fields, api
from odoo.exceptions import UserError
from datetime import date

#Model of Register Payment in Sale order
class RegisterPayment(models.TransientModel):
    _name = 'register.payments.wizard'
   
    invoice_number = fields.Many2many('account.invoice', string = "Invoices")   
   
    @api.multi
    @api.onchange('invoice_number')
    def _default_invoices(self):
        sale_orders = self.env['sale.order'].browse(self._context.get('active_ids'))
        check_invoice_address = len(set(sale_orders.mapped(lambda rec:rec.partner_invoice_id.id)))
        if check_invoice_address != 1:
            raise UserError("Can to Initiate payment for different Invoice Address.")
        invoices = []
        for sale_order in sale_orders:
            invoices.append(sale_order.invoice_ids.ids)
        self.invoice_number =  sum(invoices,[])

How to implement it without using onchange and default in odoo12 community version

Please help.

Thanks