asecondwill said over 2 years ago on Embedding Youtube Videos into Action Text with Stimulus :
I think it's because when you do things like 

document.querySelector('[data-trix-action="embed"]').addEventListener("click", event => {

It is setting that on the whole document, not just elements within the controller, right? 

So need to replace 
With something more specific.  Any hints what?   this.element does take querySelector