https://github.com/Kashdeya/Tiny-Progressions/blob/master/src/main/java/com/kashdeya/tinyprogressions/blocks/BlockGrowthUpgrade.java#L51

this is a client only method.

It is only called in the client proxy which should keep it client side.
If it's a problem then move it? It's only added so the model won't look for all possible states for the fluid level in the json blockstates file.

If you were going to do it differently you could make the state mapper method in an interface and implement it to your block then call the method with an @SideOnly(Side.CLIENT) annotation.

*Edit - I'll fork the repo later on and make a PR to tidy up the experimental code I sent over to Kash (it has unnecessary casts and stuff in it). It was done 'on the fly' to see if it stopped the crashes.

*Edit2 - urgh! you are using the item model mesher to register your items and item blocks - wai pls? :)

Sorry I missed your comments, I had a fix last night. dark already has a test build. it was the statemapper that was actually client side only so it would crash on server start when the block init. I just moved it over to only call client side, it should be good now. Hope you hadn't done anything yet.

No worries - can you push the changes to the repo - you still have some stuff there that I would like to help out with if that is OK?

Yeah I'll get It push later tonight.