Over here you convert VillagerTrades.ItemListing[] to an ArrayList using Arrays.asList.

The issue with this is that Arrays.asList doesn't return a "true" ArrayList, and you cannot add more values to it after the fact.

This is causing an issue with CraftTweaker as someone is trying to change the Bard's trades but are unable to due to the above situation.

You can read more about the caveats of Arrays.asList here:
https://stackoverflow.com/a/158269
and some solutions to the problem here:
https://stackoverflow.com/a/13421319