Removing a kit removes the item from the TreeModel and puts it into
KitModel::m_toRemoveList. A later update to that kit would trigger
the lambda connected to the dirty signal, thus calling node->update(),
which eventually ends up calling node->parent()->m_children, and
node->parent() is a null pointer there.

Change-Id: I9006f1eea036ae99b48a47baa2bc2a33bdcb84b1
Reviewed-by: hjk <hjk@theqtcompany.com>