The EndDialog callback Hdl was disposing the Dialog, which is a little
troublesome since the stack wants to go back through the Dialog code
when the callback is done.
Rather just the more normal synchronous Execute() style of Dialog
execute, instead of the asynchronous StartExecuteModal.

Change-Id: I14933bd475da228c9648a6fa0564bda4a60d9d12
Reviewed-on: https://gerrit.libreoffice.org/36074Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>