I changed the entity render registration line with
RenderingRegistry.registerEntityRenderingHandler(EntityAntimatter.class, new RenderSnowball(Minecraft.getMinecraft().getRenderManager(), AntiItems.antimatter, Minecraft.getMinecraft().getRenderItem()));
But I'm still getting this:
[17:10:47] [main/INFO] [GradleStart]: username: Mutantoe
[17:10:47] [main/INFO] [GradleStart]: Extra: []
[17:10:47] [main/INFO] [GradleStart]: Running with arguments: [--userProperties, {}, --assetsDir, C:/Users/Joe/.gradle/caches/minecraft/assets, --assetIndex, 1.8, --accessToken, {REDACTED}, --version, 1.