[FIX]mail: detach non existing element
[FIX]mail: detach non existing element

This commit fixes a js crash that occured when the user was trying to
edit a new record while the chatter was not completely loaded
(this.field.thread.$el was not in the dom yet)

Behavior before this fix:
Traceback js & impossible to edit the form afterward.

Behavior after this fix:
no traceback, still possible to edit the form normally.
The thread will display on next chatter re-render. Could be forced by
triggering an onchange.

opw-1986061

closes odoo/odoo#33381

Signed-off-by: Alexandre Kühn (aku) <22220c18caede9806cb42c8e2b41136d711673bf@odoo.com>
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo