While I was experimenting making an Eldritch Disciple, I added hellfire warlock after I completed Eldritch Disciple. Immediately the tab for adding invocations disappeared. I could bring the tab back when I deleted Hellfire Warlock. Problem is I have no idea on how exactly to reproduce this error. I have encountered the problem before and sometimes it resolves itself when I reload Hero Lab and other times it doesn't. Other times it never happens. I have no idea what is causing the problem. You might want to check out how Hellfire Warlock interacts with builds that are not Warlock with Binder dips. I am starting the think the prestige class has a bug or two under the hood.
Ok, I don't know if I fixed it, but I did find some suspicious code which I have removed. In its place, I gave the Hellfire Warlock the ability to improve one class's spellcasting ability since that works for Warlocks now.
One last thing, I don't know if this has been mentioned before, but Paragnostic Apostle has a prereq bug related to Knowledge: Other.
That's correct. It's Knowledge (any other) as in any other knowledge besides Religion or Arcana.