Getting Nil Exception on RaiseEvent

I have a listbox subclass to which I have added a Refresh event. At a certain moment the instance listboxes need to refresh their contents, and some do it slightly differently. So each listbox adds itself to an array, and the refresh sequence is triggered using a subclass method which does nothing but RaisEevent. I have, however, had a couple of instances of NilObjectException. I capture the stack on such occasions, part of which is shown below:

01 Sep 2020 16:04:20 FATAL: Error - fatal exception ‘NilObjectException’, number 0
01 Sep 2020 16:04:20 FATAL: Reason:
01 Sep 2020 16:04:20 FATAL: Message:
01 Sep 2020 16:04:20 FATAL: Stack: RuntimeRaiseException
01 Sep 2020 16:04:20 FATAL: RaiseNilObjectException
01 Sep 2020 16:04:20 FATAL: Delegate.IM_Invoke%%o
01 Sep 2020 16:04:20 FATAL: AddHandler.Stub.43%%
01 Sep 2020 16:04:20 FATAL: MailboxDisplay.raiseRefresh%%o
01 Sep 2020 16:04:20 FATAL: Operation.refreshMailboxtables

Anyone know what this might be about? I’m using 2020r1 under Mojave.

I also caught this once in the debugger; the debugger stopped on the RaiseEvent statement.