This patch fixes crashes that we've noticed during migration from Qt5 to
Qt6 in recent months.
If you select items in a tree in a specific direction or range (all, or
from bottom to top) and delete them, there is a high change user will
experience a crash in `testStatus` function. This problem arises because
we're getting into use-after-free situation.
Looking at the callstack there are a lot of calls to
`itemSelectionChanged` during deletion, which takes over item creation
after deletion in `TreeWidget::_slotDeleteObject`. This in turn causes
`DocumentObjectItem::testStatus` to be called prematurely when we have
dangling pointers in object map still. `itemSelectionChanged` signal is
being transmitted because the selection range is changing as we're
constantly deleting and readding certain items.
Previously there was `blockSelection` call during deletion, but it turns
out the signals can still be emitted even AFTER we delete the item. This
had to somehow change between Qt5 and Qt6. So, to be safe, move the
signal block for selection before the obj deletion loop to be sure we
won't retransmit this signal during an uncertain state.
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