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/8770
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>