Before this, the resource_ref field is computed once in the default_get.
This means if the cache is invalidated, its value is unrecoverable.
Notably, the cache is invalidated when an attachment is generated
(triggering a commit).
So calling _generate_template could invalidate resource_ref,
which we use as a key to retrieve its result.
This results in a traceback for the user when trying to preview
a template where the first record hasn't yet generated its attachment.
As an example, trying to preview `Sales: Send Quotation`
on fresh installs.
task-3162320
X-original-commit: b4bd93c9b61736a7f41d70afe5db977a6e4b35e0
Part-of: odoo/odoo#118710
Renaud Thiry
5c146ec219