public static void addPlayerSkinTypeRenderer(String type, RenderPlayer renderPlayer) { ClientProxy.MINECRAFT.getRenderManager().getSkinMap().put(type, renderPlayer); }

will cause a crash when used as getSkinMap() returns an unmodifiable map

public Map<String, RenderPlayer> getSkinMap() { return (Map<String, RenderPlayer>) java.util.Collections.unmodifiableMap(skinMap); }