looker vanilla javascript

i uploaded a bundle.js file to my project . so now i have 3 files …

Model_file.model

bundle.js

manifest.lkml

in the bundle i have some simple vanilla javascript

document.body.innerHTML = document.body.innerHTML + "<div>Test Extension</div>";

when i try to view the extension from the Browse menu .. it gives me an error

Cannot connect to extension

and there are no javascript errors in the developer console

any assistance is appreciated

thanks