In mass mailing, when an iframe is loaded, the `contentDocument.body` could be `undefined` this caused a traceback when trying to observe the iframe. Observing the `contentDocument` is safer as it will always be defined. task-2869490 closes odoo/odoo#92630 X-original-commit: 00a2843216128df6454d5c6ced3a34cd02e7d6d9 Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>