message: only detach if created before
diff: @@ -55,9 +55,11 @@ export default class MainSplit extends React.Component {
  }
  componentWillUnmount() {
  + if (this.resizer) {
  this.resizer.detach();
  this.resizer = null;
  }
  + }
  componentDidUpdate(prevProps) {
  const wasExpanded = !this.props.collapsedRhs && prevProps.collapsedRhs;
  