Backport bug 1989840 - Don't call unwrap() in webext-storage custom types

[bendk]

Backporting the fix for https://bugzilla.mozilla.org/show_bug.cgi?id=1989840 to the desktop-140 branch.

I think we should uplift to the ESR branch since this is a hard crash and the extension devs are saying that users are reporting issues to them.

Also, please double-check my git rev logic: The desktop ESR branch has 9b46be5beedb6a1d859014a71bac58e2d722f954 as the app-services rev so I used that to create the desktop-140 branch.

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