[FIX] mass_mailing: get correct editable in mass mailing commit changes.

Before this commit:

While saving the body_html a traceback occurred regarding reading HTML
in commit changes as it could not find the editable.

After this commit:

Now while saving the body_html the commit changes could read HTML in editable.

task-3496863

closes odoo/odoo#134614

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>