What does "uncaught illegal access" really mean?

Folks, I’m trying to get to the bottom of why secondary windows support doesn’t work on electron. One of the differences is that we get a “uncaught illegal access” message in the browser log when closing the secondary window.
The trouble is that we can’t catch this error in the debugger, so I’m poking around to see what’s happening.

The problem I’m facing is that the internet does not seem to have an answer what “illegal access” really means: is it some kind of “Null pointer exception” or does it have to do with access to parts of the DOM you’re not allowed to? Knowing exactly what to look out for would help in my search of the culprit.