[FIX] mrp : prevent error when creating new record of mrp.workorder in tree view

while we are creating a new record of the mrp.workorder from the editable list
view (mrp_production_workorder_tree_editable_view), we are getting the singleton
error, as it is not taking any production record.

by applying this commit, it will resolve that singleton error

sentry - 3982694755

closes odoo/odoo#115862

X-original-commit: 91e7d36a48d58aea1d63999dd150b543983aeba1
Signed-off-by: Tiffany Chang <tic@odoo.com>

addons/mrp/models/mrp_workorder.py

@@ -433,9 +433,10 @@ class MrpWorkorder(models.Model):
 
     @api.onchange('finished_lot_id')
     def _onchange_finished_lot_id(self):
-        res = self.production_id._can_produce_serial_number(sn=self.finished_lot_id)
-        if res is not True:
-            return res
+        if self.production_id:
+            res = self.production_id._can_produce_serial_number(sn=self.finished_lot_id)
+            if res is not True:
+                return res
 
     def write(self, values):
         if 'production_id' in values and any(values['production_id'] != w.production_id.id for w in self):