fix getAttribute exception in notes plugin #3285

plugin/notes/plugin.js

@@ -148,7 +148,7 @@ const Plugin = () => {
 		// Look for notes defined in an aside element
 		if( notesElements ) {
 			messageData.notes = Array.from(notesElements).map( notesElement => notesElement.innerHTML ).join( '\n' );
-			messageData.markdown = typeof notesElements[0].getAttribute( 'data-markdown' ) === 'string';
+			messageData.markdown = notesElements[0] && typeof notesElements[0].getAttribute( 'data-markdown' ) === 'string';
 		}
 
 		speakerWindow.postMessage( JSON.stringify( messageData ), '*' );