I added by accident an empty issue. The next time I
used a form
that listed issues in a menu, I got an exception.

In (Multi)linkHTMLProperty.menu you set 'option' to be
the labelproperty of the objects linked to (which, in
my case, was
accidentally left empty).

Then you assign it to 'lab', and call 'len(lab)', so I
get an
exception since you take the length of 'None'...

I'd suggest to replace

option = linkcl.get(optionid, k)

with

option = linkcl.get(optionid, k) or ''