[FIX] sale_management: Pricelist singleton error while choosing optional product

Backport of b6ced2e13e328539b086067a836d1e0fba3990b7

opw-2517635

closes odoo/odoo#73320

Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>

addons/sale_management/models/sale_order.py

@@ -181,7 +181,8 @@ class SaleOrderOption(models.Model):
         new_sol = self.env['sale.order.line'].new(values)
         new_sol._onchange_discount()
         self.discount = new_sol.discount
-        self.price_unit = new_sol._get_display_price(product)
+        if self.order_id.pricelist_id and self.order_id.partner_id:
+            self.price_unit = new_sol._get_display_price(product)
         return {'domain': domain}
 
     @api.multi