Here is a patch that appears to fix the issue (tested for the problem
cases with 1.5.0, though the diff is generated against current hg sources).

Although the negative multilink value not being a member of the class is
what causes the templating error, it seems the actual problem starts
earlier; the emailed exception information includes another error (a
normal one that would have been reported to the user if the templating
error hadn't arisen and so resulted in an email to the admin).  This is
a "not currently an element" error that there is an attempt to remove a
multilink value despite that value not being an element of the multilink
in the first place (it's not an element in the database; it is an
element in the results of the first erroneous edit).

Because that case can occur legitimately, I think the error is
incorrect, so this patch just replaces it with "pass".  (If the code is
operating correctly, ignoring an attempt to remove something that's not
there to remove in the first place seems completely harmless and in line
with what the user presumably intended by the removal.)

This fixes both cases of this bug that I know of.  (In addition to the
one I described in the original submission, I've since become aware of
the following case of the same bug: the original submission trying to
create a database item, with nonempty value for a multilink, has an
error.  A resubmission - presumably trying to correct the error - then
changes the multilink value to "no selection".  This resulted in the
same templating error before this patch, and works fine after the patch.)