[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) <aku@odoo.com>

addons/mail/static/src/js/chatter.js

@@ -139,7 +139,7 @@ var Chatter = Widget.extend({
         if (this.fields.activity) {
             this.fields.activity.$el.detach();
         }
-        if (this.fields.thread) {
+        if (this.fields.thread && this.fields.thread.$el ) {
             this.fields.thread.$el.detach();
         }
 
@@ -398,7 +398,7 @@ var Chatter = Widget.extend({
                     self.fields.followers.$el.insertBefore(self.$('.o_chatter_button_attachment'));
                 }
             }
-            if (self.fields.thread) {
+            if (self.fields.thread && self.fields.thread.$el) {
                 self.fields.thread.$el.appendTo(self.$el);
             }
         }).always(function () {