It looks like dataRowsController is no longer a valid object by the
time -addObject:/-insertObject: is called on it.
Thanks for the reply Dave.
That doesn't seem to be the problem. dataRowsController is hooked up
via Interface Builder to the NSArrayController. I know it's valid
because the method in my code [dataRowsController selectionIndex]
works, as does a test method [dataRowsController content].
I think I've found the problem though.
It seems that the addObject: or insertObject: method call on the
controller actually calls the getter then the setter for the whole
array. I had an NSLog in my getter that was actually causing the error: