Reusing the same instance will, in the following case, lead to a
crash. It appears that the CertificateChooser is getting disposed
somewhere as mpDialogImpl in its base class ends up being null:
1. Create an empty Writer document and add a digital signature
in the Digital Signatures dialog
2. File > Save As the document, check the "Encrypt with GPG key"
checkbox, press Encrypt, and crash in Dialog::ImplStartExecute()
Change-Id: I9aaa1bd449622e018b502d68c53d397255a1b61a
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/163065
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>
Reviewed-by: Patrick Luby <guibomacdev@gmail.com>