[IMP]account_payment_order: Add activities to account.payment.order model

account_payment_order/models/account_payment_order.py

@@ -13,7 +13,7 @@ from odoo.exceptions import UserError, ValidationError
 class AccountPaymentOrder(models.Model):
     _name = "account.payment.order"
     _description = "Payment Order"
-    _inherit = ["mail.thread"]
+    _inherit = ["mail.thread", "mail.activity.mixin"]
     _order = "id desc"
     _check_company_auto = True
 

account_payment_order/views/account_payment_order.xml

@@ -132,6 +132,7 @@
                 </sheet>
                 <div class="oe_chatter">
                     <field name="message_follower_ids" widget="mail_followers" />
+                    <field name="activity_ids" widget="mail_activity" />
                     <field name="message_ids" widget="mail_thread" />
                 </div>
             </form>