Description of the issue:

https://github.com/BluSunrize/ImmersiveEngineering/blob/1.16/src/main/java/blusunrize/immersiveengineering/common/blocks/metal/MixerTileEntity.java#L173

You can see below in the code that its checking if the fluid is not null before doing tank checks so it is possible that right there is null.

Crashlog:

https://pastebin.com/TFd8GzpN

Versions & Modlist

ImmersiveEngineering-1.16.4-4.1.2-129