Fix #269: TypeError: observerHandle is null in Firefox

[TrebuhD]

This fixes the linked issue, but I'm not 100% sure it won't break anything under the hood.

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[TrebuhD]

I signed it!

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CLAs look good, thanks!

[valdrinkoshi]

I'll close this for now, as we're working with @TrebuhD to provide details on the bug in #269.
The fix will require unit tests in order to avoid future regressions

[TrebuhD]

Maybe we should consider reopening this in light of the conversation over at #269 and quite a few people reporting being affected by this bug?