William Lane Craig says he doesn't think aseity is threatened by middle knowledge, because he is an anti-realist with respect to abstract objects including possible worlds. In other words, he views possible worlds as non-existent. Thus, God's middle knowledge is not dependent on something outside himself.
While that's an understandable response, it runs smack into the grounding objection (discussed in more detail here). By definition, middle knowledge is neither based on God's nature (or else it would be natural knowledge) nor based on God's volition (or else it would be free knowledge).
So, either what is called middle knowledge is based on something in God himself (in which case it is really free or natural knowledge, and there is no middle knowledge as such) or middle knowledge is based on something outside God (in which case we have the aseity problem). It does not seem possible that grounds could be something that is outside God but that doesn't exist, since - by definition - nothing meets that definition.