Error when use method app.modal.show

I have issue 'Cannot read property 'show' of undefined' when use app.modal.show(new LoginModal()).

You need to give us more information.
Such as did you import

import { extend } from 'flarum/extend';
import app from 'flarum/app';

or make sure you export your Modal Module as "LoginModal".

i forget put app.modal in extend. Thanks Red !