[rt-users] Shredder User plugin mangling tickets?

[Mathew Snyder]

We have roughly 9500 illegitimate users that were created by incoming spam.
I've been doing my best to eliminate these users by way of the Shredder Users
plugin.  For the most part, this has been a successful operation.  However, due
to the number of legitimate users, picking out the spam users is not a perfect
science and I've inadvertently removed legitimate users.

This has resulted in a ticket getting mangled...I think.  I'm not sure why it
would cause the transactions to get messed up or even if it has.  But this is
the output generated on the latest ticket to suffer the problem:

error:
Can't call method "Name" on an undefined value at
/usr/local/rt-3.6.1/lib/RT/Transaction_Overlay.pm line 690.

context:
...  	
686:  	AddWatcher => sub {
687:  	my $self = shift;
688:  	my $principal = RT::Principal->new($self->CurrentUser);
689:  	$principal->Load($self->NewValue);
690:  	return $self->loc( "[_1] [_2] added", $self->Field,
$principal->Object->Name);
691:  	},
692:  	DelWatcher => sub {
693:  	my $self = shift;
694:  	my $principal = RT::Principal->new($self->CurrentUser);
...  	

code stack:
/usr/local/rt-3.6.1/lib/RT/Transaction_Overlay.pm:690
/usr/local/rt-3.6.1/lib/RT/Transaction_Overlay.pm:602
/usr/local/rt-3.6.1/share/html/Ticket/Elements/ShowTransaction:54
/usr/local/rt-3.6.1/share/html/Ticket/Elements/ShowHistory:102
/usr/local/rt-3.6.1/share/html/Ticket/Display.html:63
/usr/local/rt-3.6.1/share/html/autohandler:279

The ticket displays a few of the transactions on the ticket then the above error.

Is this a result of the Requestor being removed as a suspected spam created user
or the indication of something else?  I've removed roughly 8500 users already
and have only seen this happen once before.  If there is no cure for it I'm not
too concerned as the nature of the method of removing these users is error prone
and I've made that clear to my superiors.  However, if there is a way to avoid
this, I would love to know.

Thanks,
Mathew