This doesn't seems to happen when I'm not logged in and only when using gadgets that use wbEntityId.
Seems like https://www.wikidata.org/wiki/User:Underlying_lk/reasonator.js is causing this.
I think that I experience these issues, but I'm not loading the script reasonator.js. I wonder if this is really a problem of the JavaScript, since my HTML code doesn't even have the string "Add a new statement" until I reload the page. Are your issues and mine different, @Sjoerddebruin?
mw.util.addPortletLink is a method of the mw.util singleton, which needs to be ensured by any code that uses it by declaring a deepndency on mediawiki.util.
When that is done, the system automatically makes sure that that is loaded before executing the rest of the code. This is working as intended.
I can not reproduce this in background tabs in Safari, Firefox or Chrome. If you see it again, please create a new task, and ideally with a "stack trace". You can find the stack trace by clicking the < triangle next to the error message. This will help us find which extension or which gadget is causing the problem.