Not sure if you've done anything to fix it when upgrading to even newer versions, but when I updated a world from build 80 to build 84, the danknulls in my inventory became much less "dank" and much more "null".
what happened:
After updating, Danks in my inventory became null items and throw an exception if selected or interacted with
What I expected to happen:
the item to not become unusable, be internally migrated if any internal name changes or anything like that were needed, etc. etc. Basically, don't break old worlds?
Reproduction steps:
update to 084 (including several dependency/dependent updates)? Don't think I did anything else to cause this, so I'm surprised I'm the only one to report it.
crash report:
crash-2020-03-16_15.01.00-client.txt

And problem solved before I even finished typing up my issue report. Apparently it happens only when updating to 084. I tried updating to 089, rolling back the world to FML's backup (the one it makes when the registry has changed), and loading again... no null items?
That said, before I close this, I was curious what exactly happened to cause this in the first place? Can you explain (don't hesitate to be technical; I am an IT student trying to learn from your experience)?
After you see this, feel free to close it since obviously the core issue isn't really an issue after all.