message: Null check readMarkerEventId, update comment
diff: @@ -767,13 +767,16 @@ var TimelinePanel = React.createClass({
  },
  canJumpToReadMarker: function() {
  - // Only show jump bar if RR !== RM. If they are the same, there are only fully
  + // 1. Do not show jump bar if neither the RM nor the RR are set.
  + // 2. Only show jump bar if RR !== RM. If they are the same, there are only fully
  // read messages and unread messages. We already have a badge count and the bottom
  // bar to jump to "live" when we have unread messages.
  - // We want to show the bar if the read-marker is off the top of the screen.
  - // Also, if pos === null, the event might not be paginated - show the unread bar
  + // 3. We want to show the bar if the read-marker is off the top of the screen.
  + // 4. Also, if pos === null, the event might not be paginated - show the unread bar
  const pos = this.getReadMarkerPosition();
  - return this.state.readMarkerEventId !== this._getCurrentReadReceipt() && (pos < 0 || pos === null);
  + return this.state.readMarkerEventId !== null && // 1.
  + this.state.readMarkerEventId !== this._getCurrentReadReceipt() && // 2.
  + (pos < 0 || pos === null); // 3., 4.
  },
  /**
  