TypeError: window.getSelection() is null in Firefox, causing sporadic issues

[bashenk]

Description

Consistent error, which sporadically causes significant issues in some pages in Firefox.
The error, as shown in the stack trace below, is TypeError: window.getSelection() is null

Note: I use Firefox Portable. However, I have no reason to believe at this point it is exclusive to the portable version.

Steps to reproduce

Open Firefox with Yomichan installed and open a tab to YouTube. Error will be in the console while loading the page.
Sometimes, when browsing to a video, it will completely prevent the video from starting until I disable this add-on. However, it does appear that once a video has loaded once in that session, subsequent videos will usually load even with this enabled. Hence, I have primarily encountered it when loading a video in a new private window.

Note that the error is not exclusive to YouTube, but I haven't specifically noticed how it affects other pages.

Stack trace

15:03:13.384 Yomichan 22.10.23.0 has encountered a problem.
Originating URL: about:blank
TypeError: window.getSelection() is null
setEnabled@moz-extension://967dad89-e02d-41c4-9d90-7f188875dbb2/js/language/text-scanner.js:147:63
prepare@moz-extension://967dad89-e02d-41c4-9d90-7f188875dbb2/js/language/text-scanner.js:121:14
prepare@moz-extension://967dad89-e02d-41c4-9d90-7f188875dbb2/js/app/frontend.js:141:27
async*@moz-extension://967dad89-e02d-41c4-9d90-7f188875dbb2/js/app/content-script-main.js:51:24
async*@moz-extension://967dad89-e02d-41c4-9d90-7f188875dbb2/js/app/content-script-main.js:57:3

Issues can be reported at https://github.com/FooSoft/yomichan/issues core.js:732:35
    log moz-extension://967dad89-e02d-41c4-9d90-7f188875dbb2/js/core.js:732
    error moz-extension://967dad89-e02d-41c4-9d90-7f188875dbb2/js/core.js:754
    <anonymous> moz-extension://967dad89-e02d-41c4-9d90-7f188875dbb2/js/app/content-script-main.js:55
    <anonymous> moz-extension://967dad89-e02d-41c4-9d90-7f188875dbb2/js/app/content-script-main.js:57

Possible cause/solution

Looking into window.getSelection() online seems to show that this is something that has affected Firefox for a very long time.
See this stackoverflow answer for further information:
https://stackoverflow.com/questions/20419515/window-getselection-of-textarea-not-working-in-firefox/20427804#20427804

Browser version

Firefox (64-bit) 106.0.5 and 107.0
And some earlier versions, before I decided to submit the bug report

Yomichan version

22.10.23.0 and likely some earlier versions

[TomPlum]

I'm getting the same in Firefox and Chrome. Can't add cards to Anki, the button is greyed out and the console consistently shows the same error.