This is a discussion topic for the Ore project, AntiShulkerDispenserCrash. View the full project on Ore for downloads and more information.
There is a bug that allows the dispenser at y=255 with up facing or y=0 with down facing to crash the server when dispensing a shulker box with item(s). This plugin was created to avoid this bug from crashing the server.
This plugin will print a warning message when a dispenser like above is activated and cancel the event. Other than that it doesn’t do anything.
See README.md on GitHub repo for details.