In addition to having a null messageManager it's also possible the target is already a dead wrapper if the browser has been closed.
Felipe it looks like you've reviewed this code before, but feel free to redirect.
Attachment #8786900 - Flags: review?(felipc)
Attachment #8786900 - Flags: review?(felipc) → review+
https://hg.mozilla.org/integration/mozilla-inbound/rev/6435e739a7ff8a1d9e9a3f6efccc34d4eb620f6d Bug 1299592 - Handle dead windows in ContentSearch.jsm. r=felipe
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