message: Move error dialog to the new check
  So we just check for an existing call in one place where it doesn't race
diff: @@ -754,7 +754,15 @@ export default class CallHandler extends EventEmitter {
  console.log("Current turn creds expire in " + timeUntilTurnCresExpire + " ms");
  const call = MatrixClientPeg.get().createCall(mappedRoomId);
  + try {
  this.addCallForRoom(roomId, call);
  + } catch (e) {
  + Modal.createTrackedDialog('Call Handler', 'Existing Call with user', ErrorDialog, {
  + title: _t('Already in call'),
  + description: _t("You're already in a call with this person."),
  + });
  + return;
  + }
  if (transferee) {
  this.transferees[call.callId] = transferee;
  }
  @@ -806,14 +814,8 @@ export default class CallHandler extends EventEmitter {
  return;
  }
  - console.log("getting call for room " + room.roomId);
  - if (this.getCallForRoom(room.roomId)) {
  - Modal.createTrackedDialog('Call Handler', 'Existing Call with user', ErrorDialog, {
  - title: _t('Already in call'),
  - description: _t("You're already in a call with this person."),
  - });
  - return;
  - }
  + // We leave the check for whether there's already a call in this room until later,
  + // otherwise it can race.
  const members = room.getJoinedMembers();
  if (members.length <= 1) {
  