the search_popup macro in the classic template is hardcode to display a
popup search dialog for issues only.

To fix, simply replace the following on (or about) line 234:
  <span tal:replace="structure python:db.issue.classhelp(columns,

with:
  <span tal:replace="structure python:db[db_klass].classhelp(columns,

This should allow users to use the 'db_klass' variable to change the
class of the classhelper (similar to other search_* macros).