add null check

src/client/js/components/PageEditor/DrawioModal.jsx

@@ -89,8 +89,11 @@ export default class DrawioModal extends React.PureComponent {
         const parser = new DOMParser();
         const dom = parser.parseFromString(event.data, 'text/xml');
         const value = dom.getElementsByTagName('diagram')[0].innerHTML;
+
+        if (this.props.onSave != null) {
         this.props.onSave(value);
       }
+      }
 
       window.removeEventListener('message', this.receiveFromDrawio);
       this.hide();