When calling insertRows(), the model of the child items was not
correctly updated. insertRow() is behaving correctly.
Pick-to: 6.9 6.8
Fixes: QTBUG-131372
Change-Id: I8b6aef7ab97887c6eb46eb21b992d76e4d59b3a0
Reviewed-by: David Faure <david.faure@kdab.com>