[FIX] web_editor: prevent traceback when custom color area is excluded

Bug since the combination of [1] and [2].

[1]: https://github.com/odoo/odoo/commit/1c1894d6d976806be6951db54a3b741dfd093ece
[2]: https://github.com/odoo/odoo/commit/8b5640b46a33fec5fdac4b19ea354e67211e8e46



task-2464340

closes odoo/odoo#67364

X-original-commit: d7a540c545cfe2500b08f5d6dfb6fe99747cc7de
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>