Cannot read property 'querySelector' of undefined

This error message usually appears when you try to call the `querySelector` method on a variable that is undefined.

For example, if you have a variable `element` that you expect to reference a DOM element, but it is not properly initialized or assigned, then when you try to call `element.querySelector()`, you'll get the "Cannot read property 'querySelector' of undefined" error.

To fix this error, make sure that your variables are properly initialized or assigned before attempting to call methods on them. You may also want to check if the element you're trying to query actually exists on the page.