Just tried loading up a test instance of 1.8.9, and received this crash.

Traced the code back a little bit, looks like it's this line specifically that causes the crash. I don't imagine it's a big deal, but figured you'd want it - I'm on the closed beta of Thaumcraft 5.1 by the way, I don't know if that could be the cause or not, but it seems like this is on Botania's side, unfortunately.

Let me know if I can help any other ways!