If you try to open a record in the basic MARC editor by passing bad or deleted biblionumber you get an error: Can't call method "title" on an undefined value at /usr/share/koha/intranet/cgi-bin/cataloguing/addbiblio.pl line 827. We should show a friendlier error message to the user.
Just tested with: cgi-bin/koha/cataloguing/addbiblio.pl?biblionumber=10000000 Received a nice error: The record you are trying to edit doesn't exist. Add a new record or do a catalog search.