The objects are still referenced in the collection you get
from self selected. The line with "each := nil" is useless
as each is only a temporary variable. I assume that you want
to empty the selected collection as well. you could do

DataManager>>deleteSelectedAbstractors
self selected copy do: [:each |
self abstractors remove: each.
self selected remove: each.
]

So...why would I use a copy (self selected copy) in this case? Is that a clue to my misunderstanding?

Rob