message: Prevent RoomDirectory from exploding when filterString is wrongly nullified
diff: @@ -370,7 +370,7 @@ export default class RoomDirectory extends React.Component<IProps, IState> {
  private onFilterChange = (alias: string) => {
  this.setState({
  - filterString: alias || null,
  + filterString: alias || "",
  });
  // don't send the request for a little bit,
  @@ -389,7 +389,7 @@ export default class RoomDirectory extends React.Component<IProps, IState> {
  private onFilterClear = () => {
  // update immediately
  this.setState({
  - filterString: null,
  + filterString: "",
  }, this.refreshRoomList);
  if (this.filterTimeout) {
  