unblock method
Removes a User being a recipient of this chat from the blocklist.
Only meaningful, if this chat is a dialog.
Implementation
Future<void> unblock() async {
  try {
    if (user != null) {
      await _userService.unblockUser(user!.id);
    }
  } on UnblockUserException catch (e) {
    MessagePopup.error(e);
  } catch (e) {
    MessagePopup.error(e);
    rethrow;
  }
}