Per this answer, in Forward the Foundation Hari Seldon had determined that his Foundation plan for humanity had the flaw of being too human-centric, operating under the assumption that there was no other sentient life in the universe.
This opposing answer suggests that Hari did not figure this out, as if he had then he would have built that into his plan in some way when he established the two Foundations.
Due to the nature of psychohistory it is unlikely that the first Foundation would have had any knowledge of this, therefore it would have had to have been up to the Second Foundation to protect humanity from external sentient threats. We see this sort of take place in Foundation and Empire with the Mule, although that was still human against human.
Is there any direct evidence that suggests that the Second Foundation had knowledge of the flaw in the Seldon Plan? Which would then be able to suggest whether Hari Seldon himself had actually figured it out?