Hello, just posting an update here that I am working on an updated demo scene for the boundary system which should fix the problems in the demo.
When updating the plugin over time I managed to break the boundary demo and never got around to fixing it, hopefully when the demo is updated and working it will resolve your issues.
Given the error you've shown I'd say in the meantime it might help to double check for typos in both your spawn and unspawn boundary name fields within the AddAutoHandler plugin commands. I encountered the same exact error when making a typo in the unspawn. Ensure that your AutoHandler Boundary Name matches your Boundary Name, and also ensure that if you're using an unspawn boundary that the correct name is placed within the Unspawn settings as well (this should be a name of a spawn boundary, it tells the unspawn boundary what events it's allowed to unspawn.)