When rapidly left clicking schematics, or double clicking them, while the designer is open, the game crashes with the following log.
Crash log: https://pastebin.com/c2DRp82C

tested in both a clean test pack, and the dw20 1.12.2 pack, using both forge 2551 and 2531
with automated redstone: 6.0.3 and CD lib: 6.0.4

Minecraft's "double click pick all" feature somehow made my blocks try sending state synchronization packets to the client even when the call itself was already happening on client side.

fixed with CD4017BE_lib-6.0.5