2D: Fix crash with stale barsets

In the componentComplete, the list of children is iterated. If a child
is a barset, it's added to the series. If a QBarModelMapper calls
deleteLater for undetached barset before the componentComplete is called
for a barseries, it tries to qobject_cast a deleted object. The patch
sets a barset's parent to nullptr before the call to deleteLater.

Pick-to: 6.12
Change-Id: Id29e70b0d871e6b9a27c977bacd0ef6d4c61e2fb
Reviewed-by: Tomi Korpipaa <tomi.korpipaa@qt.io>