[FIX] mass_mailing: should not crash when loading snippets

Current behavior before fix:

In mobile view when we click invite button on events module which loads the
snippet, at that time there was a traceback.

Desired behavior after fix:

Now the snippet gets loaded without any traceback.

Task-2994261

closes odoo/odoo#101238

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/mass_mailing/static/src/js/mass_mailing_html_field.js

@@ -210,7 +210,7 @@ export class MassMailingHtmlField extends HtmlField {
 
         if (device.isMobile) {
             $snippetsSideBar.hide();
-            this.$content.attr('style', 'padding-left: 0px !important');
+            this.wysiwyg.$iframe.attr('style', 'padding-left: 0px !important');
         }
 
         if (!odoo.debug) {