Using class statement inside .mjs module leads to import failure

class Point {
    constructor(x = 0, y = 0) {
        this.x = x;
        this.y = y;
    }
    plus(other) {
        return new Point(this.x + other.x, this.y + other.y);
    }
    toString() {
        return `${this.constructor.name} { x: ${this.x}, y: ${this.y} }`;
    }
}

function main() {
    const p = new Point(1, 2);
    console.log('main()', p.plus(new Point(3, 4)));
}

export { main };

import QtQuick 2.12
import QtQuick.Window 2.3
import './app.mjs' as App

Window {
    title: qsTr('ECMAScript 7 Test')
    width: 640
    height: 480
    Component.onCompleted: {
        App.main() // TypeError: Property 'main' of object TypeError: Type error is not a function
    }
}