CiviReport JavaScript error: document.getElementById("custom_8_op") is null

Encountered on 4.0.7, replicated on d7.demo.civicrm.org r36786.

Steps to replicate:
1. Add an Autocomplete-Select field to CF set Constituent Information, marked searchable.
2. In FIrefox with Error Console open, visit Constituent Report (Summary) /civicrm/report/instance/1?reset=1

Result:
Error console shows error:
document.getElementById("custom_8_op") is null
where 8 is the id of the Autocomplete-Select field.

The attached patch fixes it for me in 4.0.7, following the same logic that is used to decide whether to display the _op element.