diff -r 34716fd837a4 -r be944660c56a wp/wp-admin/js/widgets/custom-html-widgets.js
--- a/wp/wp-admin/js/widgets/custom-html-widgets.js	Tue Dec 15 15:52:01 2020 +0100
+++ b/wp/wp-admin/js/widgets/custom-html-widgets.js	Wed Sep 21 18:19:35 2022 +0200
@@ -356,7 +356,7 @@
 			return;
 		}
 
-		idBase = widgetForm.find( '> .widget-control-actions > .id_base' ).val();
+		idBase = widgetForm.find( '.id_base' ).val();
 		if ( -1 === component.idBases.indexOf( idBase ) ) {
 			return;
 		}
@@ -446,9 +446,15 @@
 			});
 
 			// Accessibility mode.
-			$( window ).on( 'load', function() {
+			if ( document.readyState === 'complete' ) {
+				// Page is fully loaded.
 				component.setupAccessibleMode();
-			});
+			} else {
+				// Page is still loading.
+				$( window ).on( 'load', function() {
+					component.setupAccessibleMode();
+				});
+			}
 		});
 	};