Investigate NoScript related errors in RequestGuard.js

While testing pre-alpha candidates I saw numerous errors related to NoScript's RequestGuard.js. We got a report in our blog as well (https://blog.torproject.org/comment/275906#comment-275906)

JavaScript error: moz-extension://{NoScript UUID}/bg/RequestGuard.js, line 280: Error: Could not establish connection. Receiving end does not exist.

We should investigate what's up with that and whether that's an issue on our end or dangerous in general.

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