[FIX] purchase: Fix unknown field '_invoice_payment_ref'

closes odoo/odoo#48008

X-original-commit: 3f7da29c850740d7b45dda5210d9fa612b4405cb
Signed-off-by: oco-odoo <oco-odoo@users.noreply.github.com>
Signed-off-by: Laurent Smet <smetl@users.noreply.github.com>

addons/purchase/models/account_invoice.py

@@ -61,9 +61,9 @@ class AccountMove(models.Model):
         refs = [ref for ref in refs if ref]
         self.ref = ','.join(refs)
 
-        # Compute _invoice_payment_ref.
+        # Compute invoice_payment_ref.
         if len(refs) == 1:
-            self._invoice_payment_ref = refs[0]
+            self.invoice_payment_ref = refs[0]
 
         self.purchase_id = False
         self._onchange_currency()