getSignInUiShownCallback() {
    return /** @type {?function(?string)} */ (
        this.getCallbacks_()['signInUiShown'] || null);
  }
Returns the signInUiShown callback. It is triggered when the sign-in UI
is shown. The tenant ID is passed.
@return {?function(?string)} The signInUiShown callback, null if not
    available.