cs.js produces "getElementById(...) is null" on all pages

[Xirema]

Mostly the error seems to be benign, but I've had at least one case (strangely enough the gateway web app of my modem) where this error causes the page to render incorrectly, so I've had to disable the addon.

This error gets spammed in the console for all web pages:

TypeError: document.getElementById(...) is null[cs.js:136:10](moz-extension://f2061c24-b8f6-4db7-b9bb-1db0386f7a6e/cs.js)
    <anonymous> moz-extension://f2061c24-b8f6-4db7-b9bb-1db0386f7a6e/cs.js:136
    #injectIntoIsolatedWorld resource://gre/modules/ExtensionContent.sys.mjs:678
    inject resource://gre/modules/ExtensionContent.sys.mjs:663
    injectInto resource://gre/modules/ExtensionContent.sys.mjs:534
    AsyncFunctionNext self-hosted:807

[maxstarkenburg]

As noted in #20, this error seems to break DuckDuckGo from displaying results. I'm guessing that all those document.getElementById added in afa9522 may need better error handling, not just the one on line 136.