Edit: I hid the feature, but I see I'm still getting visitors who want to slay monsters. Is there another flag I have to toggle to stop that?
The cavern is still visible too, which is fine, but I guess that's also controlled by other map data.
If the cavern is blocked off with natural walls, you could use revflood to hide them, but that won't work with constructed walls (by design). If you use tiletypes to add in natural walls, though, you could use revflood to hide the caverns again. That *might* help with the monster slayers, but I'm not sure.
I tried first by running gui/autochop, but there apparently weren't any options to exclude trees based on their products.
There is no global setting for protecting those types of trees. You have to define a burrow and set the protections on the burrow. This is true for the commandline as well. The error message you're seeing should obviously be better phrased, but the issue is that the command you ran has a syntax error. The syntax is:
autochop (protect|unprotect) <type>[,<type>...] <burrow>[,<burrow>...]
and you're missing the burrow specification at the end.
It even happens if I just type in modtools/item-trigger and no further arguments
modtools/item-trigger enables some EventManager events when it is run. My guess is that there is a crash bug in EventManager for one of those event types. What are you trying to build? This may be worth releasing a new 47.05 version of DFHack for.