Previewing templates in the "SELECT A TEMPLATE" tab of mass_mailing made one lose all their changes. This makes sure they are preserved and restored when needed. task-2778411 closes odoo/odoo#85932 X-original-commit: e96e11d6ab8e5f7a6d5da498409cc0d5fb26cad1 Signed-off-by: David Monjoie (dmo) <dmo@odoo.com> Signed-off-by: Antoine Guenet <age@odoo.com>