Just updated to the newest version of Paper #164, and the following message was thrown into console.

20.01 01:04:57 [Server] WARN @EventHandler method com.wfector.notifier.ChestShopNotifier#onChestShopTransaction returns non-void type boolean. This is an unsupported behavior which will be removed in a future version of Paper.This should be reported to the developers of ChestShopNotifier v1.3.3 (build 51), (BetaBlaze,Phoenix616,FrankHeijden)

Thanks for bringing that to my attention.