message: this is optional, and not provided for first edit
diff: @@ -36,7 +36,7 @@ export default class EditHistoryMessage extends React.PureComponent {
  static propTypes = {
  // the message event being edited
  mxEvent: PropTypes.instanceOf(MatrixEvent).isRequired,
  - previousEdit: PropTypes.instanceOf(MatrixEvent).isRequired,
  + previousEdit: PropTypes.instanceOf(MatrixEvent),
  isBaseEvent: PropTypes.bool,
  };
  