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Vincent authored
See https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/MessageSender#Type This _might_ be causing an error and, with that, #166, but I'm not sure yet. In any case, this is a potential failure case, so fixing it should not cause harm. (Also marked other optional properties as optional, but since we're not using them anywhere, those did not require code changes.)