[FIX] web_editor: solve traceback table RTL scroll

Commit that introduced the issue: fbc167bf84340b4bb6d0f8c59f2734814f56c6df

Issue:
======
Adding a table in a long chatter message with scroll raise a traceback

Steps to reproduce the issue:
=============================
- Switch to RTL lang
- Go to any form view and open the editor composer to create a log note
- Write a lot of lines so that the scrollbar appears
- Add a table
- Log the note
- Try to scroll -> traceback

Origin of the issue:
====================
The `_onScroll` method is called and it has `this._rowUiTarget` as the
row from the composer dialog which is not in the ui anymore so
`closestElement(row, 'table')`  will return `null`.

Solution:
=========
We just do nothing when the element is not connected.

task-3707808

closes odoo/odoo#151800

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

addons/web_editor/static/src/js/editor/odoo-editor/src/OdooEditor.js

@@ -2916,6 +2916,9 @@ export class OdooEditor extends EventTarget {
      * @param {HTMLTableRowElement|HTMLTableCellElement} element
      */
     _positionTableUi(element) {
+        if (!element.isConnected) {
+            return;
+        }
         const tableUiContainerRect = this._tableUiContainer.getBoundingClientRect();
         const isRtl = this.options.direction === 'rtl';
         const isRow = element.nodeName === 'TR';