Uncaught TypeError: can't access property "el", n is undefined

try to place your code inside

editor.on('load', () => {
 // ...
})

Problem "solved" removing :

const panelManager = editor.Panels;
**panelManager.getButton('views', 'open-blocks').set('active', true);**

looking for a better solution to make active the block panel af first

I've also noticed other conflict like with my custom modals and modal from export plugin (clicking export plugin was fire my custom modal on FFox), or for example panel that i've remode appeared again i've solved putting everything in my code inside

window.onload = function(){
    //my code here
}