message: Null guard TimelinePanel unmount edge
diff: @@ -559,7 +559,7 @@ class TimelinePanel extends React.Component<IProps, IState> {
  // updates from pagination will happen when the paginate completes.
  if (toStartOfTimeline || !data || !data.liveEvent) return;
  - if (!this.messagePanel.current) return;
  + if (!this.messagePanel.current?.getScrollState()) return;
  if (!this.messagePanel.current.getScrollState().stuckAtBottom) {
  // we won't load this event now, because we don't want to push any
  @@ -570,7 +570,7 @@ class TimelinePanel extends React.Component<IProps, IState> {
  }
  // tell the timeline window to try to advance itself, but not to make
  - // an http request to do so.
  + // a http request to do so.
  //
  // we deliberately avoid going via the ScrollPanel for this call - the
  // ScrollPanel might already have an active pagination promise, which
  