I've had a look into this and failed to come up with a solution, so far.
I notice a reference to another add-on, "ConvEss". It may be worth posting in the relevant discussion thread (if you haven't done so already) so that both add-on developers are aware of the error.
Things that may be worthy of note:
- This add-on extends the Conversation model with an Event Listener which uses both a hint and a $class check within the callback class (this is just a redundant but important statement I had left in in one case, prior to forgoing the check and allowing the event hints to handle this for my other extends - this is briefly mentioned by me in this thread).
- This add-on does not extend the 'invite-insert' controller route.
Is the error reproducible by inviting a user to a conversation (Invite More > Invite Members)?