Caused by bug 1365970. This happens when we call sendWhenIdle after the frameloader has started to be torn down.
https://hg.mozilla.org/integration/mozilla-inbound/rev/5e070a86daad50fa6ea1902f86bf9d29fad390bf Bug 1401422: Bail out from idle callback if frameloader is being destroyed. r=mikedeboer
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