The issue here seems more than just a few error messages.

The errors are logged because when the InvokePendingModifiedEvent function calls the pending events, it calls them without an argument, and that becomes an invalid item ID (0) in the callback. So StartModify/EndModify cannot really work with custom modified events that use arguments.

I am not sure what would be a good way to proceed with this one. Maybe just not treating the SubjectHierarchyItemModifiedEvent as a modified event, or disable StartModify/EndModify for the subject hierarchy node?

If the widget gets an EndModify event it should assume that it needs to rebuild from scratch, since there's no way to know what it missed during the batch operation.

I agree, modify event with 0 ID means batch update = rebuild the widget from scratch (but preserving selection state).

Thank you! Sounds like a conceptually easy solution. Not sure though if it's easy to implement as well because the plugin logic where this event is handled has not interacted with the model directly. I'll take a look soon.

yes, good point about selection state. Also the scroll state.