Prohibit calling convert_order on Function field

Closes #13872
(grafted from 7c043ecbd617b2b318d7e3219500c901b0b85127)

--HG--
branch : 7.0

trytond/trytond/model/fields/function.py

@@ -9,7 +9,7 @@ from trytond.i18n import gettext
 from trytond.tools import is_instance_method
 from trytond.transaction import Transaction, without_check_access
 
-from .field import Field, domain_method
+from .field import Field, domain_method, order_method
 
 
 def getter_context(func):
@@ -100,6 +100,10 @@ class Function(Field):
                 'ir.msg_search_function_missing',
                 **Model.__names__(self.name)))
 
+    @order_method
+    def convert_order(self, name, tables, Model):
+        raise NotImplementedError
+
     @getter_context
     @without_check_access
     def get(self, ids, Model, name, values=None):