How to use the scratch-gui.setAppElement function in scratch-gui

dispatch(previewActions.updateProject(id, formData, username, token));
    },
    setPlayer: player => {
        dispatch(GUI.setPlayer(player));
    },
    setFullScreen: fullscreen => {
        dispatch(GUI.setFullScreen(fullscreen));
    }
});

const ConnectedPreview = connect(
    mapStateToProps,
    mapDispatchToProps
)(Preview);

GUI.setAppElement(document.getElementById('app'));
const initGuiState = guiInitialState => {
    const pathname = window.location.pathname.toLowerCase();
    const parts = pathname.split('/').filter(Boolean);
    // parts[0]: 'preview'
    // parts[1]: either :id or 'editor'
    // parts[2]: undefined if no :id, otherwise either 'editor' or 'fullscreen'
    if (parts.indexOf('editor') === -1) {
        guiInitialState = GUI.initPlayer(guiInitialState);
    }
    if (parts.indexOf('fullscreen') !== -1) {
        guiInitialState = GUI.initFullScreen(guiInitialState);
    }
    return guiInitialState;
};

render(

let pathname = window.location.pathname;
    if (pathname.substr(-1) !== '/') {
        pathname = `${pathname}/`;
    }
    if (window.location.hash === '#editor') {
        history.replaceState({}, document.title,
            `${pathname}editor${window.location.search}`);
    }
    if (window.location.hash === '#fullscreen') {
        history.replaceState({}, document.title,
            `${pathname}fullscreen${window.location.search}`);
    }
}

// initialize GUI by calling its reducer functions depending on URL
GUI.setAppElement(document.getElementById('app'));
module.exports.initGuiState = guiInitialState => {
    const pathname = window.location.pathname.toLowerCase();
    const parts = pathname.split('/').filter(Boolean);
    // parts[0]: 'projects'
    // parts[1]: either :id or 'editor'
    // parts[2]: undefined if no :id, otherwise either 'editor' or 'fullscreen'
    if (parts.indexOf('editor') === -1) {
        guiInitialState = GUI.initPlayer(guiInitialState);
    }
    if (parts.indexOf('fullscreen') !== -1) {
        guiInitialState = GUI.initFullScreen(guiInitialState);
    }
    return guiInitialState;
};

module.exports.guiReducers = GUI.guiReducers;

const defaultProjectId = 0;

// override window.open so that it uses the OS's default browser, not an electron browser
window.open = function (url, target) {
    if (target === '_blank') {
        shell.openExternal(url);
    }
};
// Register "base" page view
// analytics.pageview('/');

const appTarget = document.getElementById('app');
appTarget.className = styles.app || 'app'; // TODO
document.body.appendChild(appTarget);

GUI.setAppElement(appTarget);

const ScratchDesktopHOC = function (WrappedComponent) {
    class ScratchDesktopComponent extends React.Component {
        constructor (props) {
            super(props);
            bindAll(this, [
                'handleProjectTelemetryEvent',
                'handleSetTitleFromSave',
                'handleStorageInit',
                'handleTelemetryModalOptIn',
                'handleTelemetryModalOptOut'
            ]);
        }
        componentDidMount () {
            ipcRenderer.on('setTitleFromSave', this.handleSetTitleFromSave);
        }

projectInfo: state.preview.projectInfo,
    projectNotAvailable: state.preview.projectNotAvailable
});

const mapDispatchToProps = dispatch => ({
    getProjectInfo: (id, token) => {
        dispatch(previewActions.getProjectInfo(id, token));
    }
});

module.exports.View = connect(
    mapStateToProps,
    mapDispatchToProps
)(EmbedView);

GUI.setAppElement(document.getElementById('app'));
module.exports.initGuiState = GUI.initEmbedded;
module.exports.guiReducers = GUI.guiReducers;
module.exports.guiInitialState = GUI.guiInitialState;
module.exports.guiMiddleware = GUI.guiMiddleware;
module.exports.initLocale = GUI.initLocale;
module.exports.localesInitialState = GUI.localesInitialState;