If you activate a piston in 1.21, your game will crash instantly. To reproduce just take a piston and activate it with a lever or something else. 

Exit-Code -1
The game crashed whilst rendering block entity
Error: java.lang.NullPointerException: Cannot invoke "net.fabricmc.fabric.api.renderer.v1.Renderer.meshBuilder()" because the return value of "net.fabricmc.fabric.api.renderer.v1.RendererAccess.getRenderer()" is null