Prevent crash when flattening ComponentKey to string with invalid user.

     * Encodes a component key as a string of the form [flattenedComponentString#userId].

     */

    public String flattenToString(Context context) {

        return componentName.flattenToString() + "#" +

                UserManagerCompat.getInstance(context).getSerialNumberForUser(user);

        String flattened = componentName.flattenToString();

        if (user != null) {

            flattened += "#" + UserManagerCompat.getInstance(context).getSerialNumberForUser(user);

        }

        return flattened;

    }

    @Override