fdo#62155 - don't crash if we can't create an instance of a calc addin.

Change-Id: I478d7777949683a91e7adf9fa6b54e9d01ced676
Reviewed-on: https://gerrit.libreoffice.org/8771
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>