message: Throw an error when trying to create a group store with falsey groupId
diff: @@ -33,6 +33,9 @@ export default class GroupStore extends EventEmitter {
  constructor(matrixClient, groupId) {
  super();
  + if (!groupId) {
  + throw new Error('GroupStore needs a valid groupId to be created');
  + }
  this.groupId = groupId;
  this._matrixClient = matrixClient;
  this._summary = {};
  