The way I understood the new listField attribute, I can populate e.g. a list of tiddlers by means of checkboxes. When I put this in a New Tiddler on https://tiddlywiki.com/
I see a list of tiddlers with checkboxes, as expected. Checking one of the boxes also works as expected, with the tiddler title appearing in the list field.
But when I check a second item, I get this:
I Tried you code on Tiddlywiki.com and noticed you are using the “list” field which is a reserved fieldname. Changing this to my-list I was no longer getting the error including for the last item.
I also observe since the list changes the current tiddler it is not appropriate to set tiddler=<<currentTiddler>>unless you want the current tiddler changed, which you seem to. You can use tiddler=<<storyTiddler>> if you just want to add to the local list.
The Bug(s)?
If you use the “list” field you get an error on checking more than one item, use another name but a quick test shows the “tags” field also fails.
If the tiddler named in the checkbox widget does not exist it fails
I think it is just that the checkbox widget will not create a missing tiddler, if you create it first, it does not throw an error.
This is unfortunate because I wanted to use it to manipulate the tags.
As a rule of thumb, the “red screen of embarrassment” always indicates a core bug. It is not supposed to be possible to crash TW just by writing wikitext (all bets are off if there are custom JavaScript modules involved of course).