The crash is in kleiders_custom_renderer (Kleiders Custom Renderer API), specifically PlayerVariablesSyncMessage.handleData. The Kleiders Custom Renderer API mod syncs custom player variables to clients via a packet that references an entity by its ID. When the referenced entity is no longer in the level (e.g. it died and got removed from the world before the sync packet was processed), level.getEntity(id) returns null, and the mod doesn't null check that before calling .getData(...) on it, straight NullPointerException, which then errors out the network task handling and can desync/kick the client. This is a mod to remedy that.
