PDF-AS NPE workaround

[iaik-jheher]

(Issue migrated from Gitlab: Internal link)

When the user clicks on the "Cancel" button after entering credentials, a "Signatur-Fehler" error message is displayed.

This is caused by PDF-AS exception handling, which causes a NPE internally, resetting the cause chain.

This is supposedly fixed in internal builds (cf. Internal link), waiting for PDF-AS release to get rid of our workaround.

Current workaround state is 75ce8c3a.