Why does Unchecked lastError value occur when creating a Firefox extension?

Question

JavaScript

Rewrote the first test extension for Firefox from the documentation ( https://developer.mozilla.org/en-US/docs/Mozilla/A... )
The extension simply adds a frame to the page: I open the google.com tab and the console, there immediately errors Unchecked lastError value: Error: ID already exists: convert Unchecked lastError value: Error: ID already exists: append Why do they occur?
document.body.style.border = "5px solid yellow";