[FIX] ContentEditableHelper: do not throw on missing node

Currently,the method `findSizeBeforeElement` will throw if called when
the selection is not targetting a node inside the ContentEditableHelper
instance.

Unfortunately, this can happen since commit d466315. If a menu is
opened and we decide to click on the composer, the click event if first
caught by the menu global listener which will close the menu and invoke
a callback that focuses the grid; this focus will alter the selection of
`document`. The event is then handled by the composer component but at
that point, the selection is not targetting it anymore.

With this revision, we change the behavior of `findSizeBeforeElement` to
return a default value of 0 in those situations as we simply don't know
how to determine the selection outside of the scope of the
ContentEditableHelper.

There is a functional drawback to this solution:
When clicking on a specific character of the composer when a menu is
opened, the selection will be set to `{ start: 0, end: 0 }`.

closes #2462

Signed-off-by: Lucas Lefèvre (lul) <lul@odoo.com>

src/components/composer/content_editable_helper.ts

@@ -199,7 +199,14 @@ export class ContentEditableHelper {
       current = it.next();
     }
     if (current.value !== nodeToFind) {
-      throw new Error("Cannot find the node in the children of the element");
+      /** This situation can happen if the code is called while the selection is not currently on the ContentEditableHelper.
+       * In this case, we return 0 because we don't know the size of the text before the selection.
+       *
+       * A known occurence is triggered since the introduction of commit d4663158 (PR #2038).
+       *
+       * FIXME: find a way to test eventhough the selection API is not available in jsDOM.
+       */
+      return 0;
     } else {
       if (!current.value.hasChildNodes()) {
         usedCharacters += nodeOffset;