fix: add null checks for pos_session references to prevent attribute errors during account and discount calculation

models/pos_session.py

@@ -12,7 +12,7 @@ class PosSession(models.Model):
         """Override to allow products without income accounts when payment methods have them configured"""
         def get_income_account(order_line):
             product = order_line.product_id
-            income_account = product.with_company(order_line.company_id)._get_product_accounts()['income'] or self.config_id.journal_id.default_account_id
+            income_account = product.with_company(order_line.company_id)._get_product_accounts()['income'] or (self and self[0].config_id.journal_id.default_account_id or False)
             
             # NEW: If no income account is found on the product, check if payment methods have income accounts configured
             if not income_account:
@@ -59,7 +59,7 @@ class PosSession(models.Model):
         if not sales:
             return data
 
-        discount_product_id = self.config_id.discount_product_id.id if self.config_id.discount_product_id else None
+        discount_product_id = self[0].config_id.discount_product_id.id if self and self[0].config_id.discount_product_id else None
 
         # Build per-order breakdown of sales keyed by (account, sign, tax tuple, tags)
         order_payment_totals = {}