HTTPS Everywhere Update Channels Preferences has TypeError in 10.0a5

Open about:addons -> HTTPS Everywhere preferences -> Preferences tab -> Update Channels.

Logged: JavaScript error: resource:///actors/RFPHelperParent.jsm, line 29: TypeError: window.RFPHelper is undefined

Originally Reported: https://blog.torproject.org/comment/289060#comment-289060

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