Comment 10 for bug 1772677

OK. FYI here is the offending source code:

    _relayout() {
        /* assume 110x110 on a 640x480 display and scale from there */
        let monitor = Main.layoutManager.monitors[this._monitorIndex];
        if (!monitor)
            return; // we are about to be removed

        let scalew = monitor.width / 640.0;
        let scaleh = monitor.height / 480.0;
        let scale = Math.min(scalew, scaleh);
        let popupSize = 110 * Math.max(1, scale);

        let scaleFactor = St.ThemeContext.get_for_stage(global.stage).scale_factor;
        this._icon.icon_size = popupSize / (2 * scaleFactor);
# ^^^^^^^^^^^ This is the St.Icon the error talks about
        this._box.translation_y = Math.round(monitor.height / 4);
        this._boxConstraint.minSize = popupSize;
    }