Issue #2811717 by tedbow, drpal, RajabNatshah, droplet, alexpott: [outsidein]...

Issue #2811717 by tedbow, drpal, RajabNatshah, droplet, alexpott: [outsidein] Uncaught TypeError: Cannot read property 'addEventListener' of null

core/modules/outside_in/js/outside_in.js

@@ -124,6 +124,9 @@
    *  True enable edit mode, false disable edit mode.
    */
   function setEditModeState(editMode) {
+    if (!document.querySelector('[data-off-canvas-main-canvas]')) {
+      throw new Error('data-off-canvas-main-canvas is missing from outside-in-page-wrapper.html.twig');
+    }
     editMode = !!editMode;
     var $editButton = $(toggleEditSelector);
     var $editables;

core/modules/outside_in/templates/outside-in-page-wrapper.html.twig

@@ -3,7 +3,9 @@
  * @file
  * Default theme implementation for a page wrapper.
  *
- * For consistent wrapping to {{ page }} render in all themes.
+ * For consistent wrapping to {{ page }} render in all themes. The
+ * "data-off-canvas-main-canvas" attribute is required by the off-canvas dialog.
+ * It cannot be removed without breaking the off-canvas dialog functionality.
  *
  * Available variables:
  * - children: Contains the child elements of the page.