Backport bug 1989840 - Don't call unwrap() in webext-storage custom types #6971

[bendk]

Backporting the fix for https://bugzilla.mozilla.org/show_bug.cgi?id=1989840 to the desktop-143 branch (release)

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

For this one, I just used the existing desktop-143 branch, which is what's listed in releases's Cargo.toml.

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