message: Null guard widget listener
diff: @@ -170,8 +170,10 @@ export default class AppTile extends React.Component {
  // Widget postMessage listeners
  try {
  + if (this.widgetMessaging) {
  this.widgetMessaging.stopListening();
  this.widgetMessaging.removeEndpoint(this.props.id, this.props.url);
  + }
  } catch (e) {
  console.error('Failed to stop listening for widgetMessaging events', e.message);
  }
  