[FIX] web_editor: solve bug when removing all columns of the footer

When removing all the columns of a block with no parent, like the
footer, a traceback was appearing because updateUIVisibility was trying
to access the properties of an inexisting parent. This commit
fixes that by first checking if the parent exists.

task-2973198

X-original-commit: 3e7ad08581295af6a986ba2b81ec25d9cd78e91d
Part-of: odoo/odoo#102525

addons/web_editor/static/src/js/editor/snippets.options.js

@@ -4430,7 +4430,7 @@ registry.sizing = SnippetOptionWidget.extend({
 
         const isMobileView = this.$target[0].ownerDocument.defaultView.frameElement.clientWidth < 768;
         const isGrid = this.$target[0].classList.contains('o_grid_item');
-        if (this.$target[0].parentNode.classList.contains('row')) {
+        if (this.$target[0].parentNode && this.$target[0].parentNode.classList.contains('row')) {
             // Hiding/showing the correct resize handles if we are in grid mode
             // or not.
             for (const handleEl of this.$handles) {