[FIX] mass_mailing: solve error on add block mailing

Issue:
======
Adding a new block in mailing in email marketing raises an error.

Steps to reproduce the issue:
=============================
- Go to email marketing
- Create a new mailing
- Create one from scratch
- Drop any block
- `TypeError : Cannor read properties of null (reading 'parentElement')`

Origin of the issue:
====================
When we drop the block in the iframe's document we trigger the `click`
event but we don't have any selection yet in the document so
`anchorNode`will be `null` thus the error.

task-3724551

closes odoo/odoo#152814

Signed-off-by: Antoine Guenet (age) <age@odoo.com>

addons/mass_mailing/static/src/js/snippets.editor.js

@@ -63,11 +63,13 @@ export const MassMailingSnippetsMenu = snippetsEditor.SnippetsMenu.extend({
         // To prevent unnecessary tab shifting, we provide a selection for this specific case.
         if (srcElement.classList.contains('o_mail_wrapper') || srcElement.querySelector('.o_mail_wrapper')) {
             const selection = this.options.wysiwyg.odooEditor.document.getSelection();
-            const parent = selection.anchorNode.parentElement;
-            if (parent) {
-                srcElement = parent;
+            if (selection.anchorNode) {
+                const parent = selection.anchorNode.parentElement;
+                if (parent) {
+                    srcElement = parent;
+                }
+                this._activateSnippet($(srcElement));
             }
-            this._activateSnippet($(srcElement));
         }
     },
     /**