message: Fix read markers
  init code needs to be a constructor or its run too late
diff: @@ -109,7 +109,8 @@ export default class MessagePanel extends React.Component {
  showReactions: PropTypes.bool,
  };
  - componentDidMount() {
  + constructor() {
  + super();
  // the event after which we put a visible unread marker on the last
  // render cycle; null if readMarkerVisible was false or the RM was
  // suppressed (eg because it was at the end of the timeline)
  @@ -165,6 +166,10 @@ export default class MessagePanel extends React.Component {
  this._showHiddenEventsInTimeline =
  SettingsStore.getValue("showHiddenEventsInTimeline");
  + this._isMounted = false;
  + }
  +
  + componentDidMount() {
  this._isMounted = true;
  }
  