Problem to add CSS in InfoBox after upgrade from 1.50 to 1.71

I just done the upgrade from 1.50 to 1.71.
I was using this code to add a css in the Iframe, but it is not working anymore.

     viewer.infoBox.frame.addEventListener('load', function () {
                    var customCssLink = viewer.infoBox.frame.contentDocument.createElement('link');
                    customCssLink.href = relativeRootUrl + '/SomePath/CSS/CesiumPopUp.css';
                    customCssLink.rel = 'stylesheet';
                    customCssLink.type = 'text/css';

Have you some tips to help me ?

Can you use git bisect (Git - git-bisect Documentation) to identify the commit that caused the error in your code?