:kmag pointed out in https://bugzilla.mozilla.org/show_bug.cgi?id=1395659#c14 > I'd probably just make getContainerForCookieStoreId and getPublicIdentityFromId throw an ExtensionError for invalid IDs. Up to you, though. We can do this however these methods are used in other places that don't appear to handle thrown exceptions. On IRC he also said: > Are they used anywhere that we don't want them to throw? > If so, I'd just add a default value argument, and have them throw if it's not passed, return it if it is. Either way this would simplify the checks for `if (!containerId) {` in toolkit/components/extensions/ext-contextualIdentities.js