the RenderHandler isn't threadsafe. The FMLClientSetupEvent is dispatched to multiple mods at the same time, which could cause a crash if you modify non threadsafe maps during the execution. A problem only occurs if multiple mods have this problem. To register your renderers I suggest to use the intended event for it (EntityRenderersEvent.RegisterRenderers). pls enqueue the registering of your screens using ParallelDispatchEvent#enqueueWork