DOM is changed while js execution is paused at a breakpoint

There’s the variable containerdom with a DIV inside which my widget is rendered. I attach a callback function to the widget’s “render” event. In the callback I call containerdom.querySelector() to get the widget element that was just rendered.

Sometimes, usually after restarting Firefox, querySelector() returns null, but if I add a conditional breakpoint when it’s null and re-inspect containerdom and call querySelector() again it returns the element this time.

    treetabledom = containerdom.querySelector('.widget-treetable-wrapper');
    if (!treetabledom) {
        console.info(containerdom.outerHTML); // missing
        debugger;
        console.info(containerdom.outerHTML); // present
        var treetabledom2 = containerdom.querySelector('.widget-treetable-wrapper');
        treetabledom = treetabledom2;
    }

What’s causing it and how to fix this?