Looks like there is a time frame within which the observer service responds [to calls with an error when in shutdown](https://searchfox.org/mozilla-central/rev/9b8ebf06145feeccf34dc126cf45b07e86556392/xpcom/ds/nsObserverService.cpp#175). I think it should better be just not possible to get an instance such that [checks like this work as expected](https://searchfox.org/mozilla-central/rev/9b8ebf06145feeccf34dc126cf45b07e86556392/js/xpconnect/src/XPCJSRuntime.cpp#789) ? But yes, this phases seems also late.
Bug 1806013 Comment 3 Edit History
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Looks like there is a time frame within which the observer service responds [to calls with an error when in shutdown](https://searchfox.org/mozilla-central/rev/9b8ebf06145feeccf34dc126cf45b07e86556392/xpcom/ds/nsObserverService.cpp#175). I think it should better be just not possible to get an instance such that [checks like this work as expected](https://searchfox.org/mozilla-central/rev/9b8ebf06145feeccf34dc126cf45b07e86556392/js/xpconnect/src/XPCJSRuntime.cpp#789) ? But yes, this phase seems also late.