Avoid a potential crash in unignoredChildren

This doesn't actually fix the source of the problem, but its harmless.
This was reproduced with tst_qcolumnview and voiceover enabled

Task-number: QTBUG-32440

Change-Id: Iad27884e1ca9194f911271c16908ef358e4b1875
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>