getSignInSuccessCallback() {
    return /** @type {?Config.signInSuccessCallback} */ (
        this.getCallbacks_()['signInSuccess'] || null);
  }
@return {?Config.signInSuccessCallback} The callback to invoke when the
    user signs in successfully. The signed in firebase user is passed
    into the callback. A second parameter, the Auth credential is also
    returned if available from the sign in with redirect response. An
    optional third parameter, the redirect URL, is also returned if that
    value is set in storage. If it returns `true`, the widget will
    continue to redirect the page to `signInSuccessUrl`. Otherwise, the
    widget stops after it returns.