GmailAdRemover: current tab selector can return null

[mkazin]

v1.0

Uncaught TypeError: Cannot read properties of null (reading 'innerText')
    at Proxy.removeAds (userscript.html?name…-893a453e5e6d:19:59)
    at Pt (<anonymous>:10:89)

userscript.html:19 is-
    let tab = document.querySelector(SELECTOR_CURRENT_TAB).innerText

This code is running on an interval and makes for a good candidate for Observer adoption when wheneverElementAppears is built.