"Add Search Engine" missing in address bar context menu after theme is changed.

Tested on Windows10 64bit , MacOS 10.14, Ubuntu 20 64bit.
Firefox Nightly 88 Affected
Firefox beta 87 (option Not available)
Firefox Release 86.0.1 (option Not available)

Steps

1. set user.js on a clean profile.
   user_pref("browser.proton.enabled", true);
   user_pref("browser.proton.toolbar.enabled", true);
   user_pref("browser.proton.urlbar.enabled", true);

2. open a tab and visit youtube.com

3. Right click on address bar

4. notice "add youtube" option is available in context menu.( its the last option)

5. Now Change the firefox theme to LIGHT theme.

6. go again to youtube

7. right click on address bar.

Actual result
add youtube is missing.

Expected result
Add youtube should still be visible

@rtestard, could you please set a priority for this here?

The default access point will be the one in the shortcut buttons, that works as expected.
We also added the context menu access point to cover two cases:

1. the user disabled top sites, so the urlbar doesn't open when clicking on it
2. the page provides more than 3 engines

The latter is not so critical, since quite uncommon, but the former may be more common (I'm not sure whether we measure how many users disabled top sites though).

I actually thought to have addressed this issue, but I didn't consider that we destroy and recreate the widget in certain cases, Customize is one of them, so after customize this stops working because the event handler I added is pointing to a dead widget.
I think I have a good enough idea of the problem to look for a solution.

https://hg.mozilla.org/mozilla-central/rev/ac1d613a06d2

The patch landed in nightly and beta is affected.
:mak, is this bug important enough to require an uplift?
If not please set status_beta to wontfix.

For more information, please visit auto_nag documentation.

This only affects Proton that is not enabled in 88.

verified fix on Firefox nightly 89.0a1 using windows10 64bit , macos 10.15 and ubuntu 20 64bit