[FIX] mail: be more defensive when previewing fields

Sometimes due to new ids / not existing records for previewing crash may
happen when rendering the preview of templates. In this commit we rewrite
a bit the computation to try to be more resistant.

Task-3093257 (Mail: The Composer Update)

Part-of: odoo/odoo#107356

addons/mail/wizard/mail_template_preview.py

@@ -71,26 +71,20 @@ class MailTemplatePreview(models.TransientModel):
         """ Preview the mail template (body, subject, ...) depending of the language and
         the record reference, more precisely the record id for the defined model of the mail template.
         If no record id is selectable/set, the inline_template placeholders won't be replace in the display information. """
-        copy_depends_values = {'lang': self.lang}
         mail_template = self.mail_template_id.with_context(lang=self.lang)
-        try:
-            if not self.resource_ref.id:
-                self._set_mail_attributes()
-            else:
-                copy_depends_values['resource_ref'] = '%s,%s' % (self.resource_ref._name, self.resource_ref.id)
+        if not self.resource_ref or not self.resource_ref.id:
+            self._set_mail_attributes()
+        else:
+            try:
                 mail_values = mail_template.with_context(template_preview_lang=self.lang)._generate_template(
                     [self.resource_ref.id],
                     self._MAIL_TEMPLATE_FIELDS
                 )[self.resource_ref.id]
                 self._set_mail_attributes(values=mail_values)
-            self.error_msg = False
-        except (ValueError, UserError) as user_error:
-            self._set_mail_attributes()
-            self.error_msg = user_error.args[0]
-        finally:
-            # Avoid to be change by a cache invalidation (in generate_mail), e.g. Quotation / Order report
-            for key, value in copy_depends_values.items():
-                self[key] = value
+                self.error_msg = False
+            except (ValueError, UserError) as user_error:
+                self._set_mail_attributes()
+                self.error_msg = user_error.args[0]
 
     def _set_mail_attributes(self, values=None):
         for field in self._MAIL_TEMPLATE_FIELDS: