fix exception when stepping backwards through code highlights #3524

plugin/highlight/plugin.js

@@ -138,7 +138,7 @@ const Plugin = {
 
 					// Scroll highlights into view as we step through them
 					fragmentBlock.addEventListener( 'visible', Plugin.scrollHighlightedLineIntoView.bind( Plugin, fragmentBlock, scrollState ) );
-					fragmentBlock.addEventListener( 'hidden', Plugin.scrollHighlightedLineIntoView.bind( Plugin, fragmentBlock.previousSibling, scrollState ) );
+					fragmentBlock.addEventListener( 'hidden', Plugin.scrollHighlightedLineIntoView.bind( Plugin, fragmentBlock.previousElementSibling, scrollState ) );
 
 				} );