Description of problem:
If I create a service dialog containing a drop-down list populated by an expression method, I am unable to edit the dialog to make the list multi-selectable. I can appear to make the change, but saving the dialog results in a 500 internal server error.

It looks like the change is saved because any attempt to subsequently save the same dialog gives the same 500 error.

Version-Release number of selected component (if applicable):
5.9.0.22

How reproducible:
Every time

Steps to Reproduce:
1. Create a service dialog containing a single drop-down list element. Make this element dynamic, and create an expression method to populate it (I used a simple expression method that lists all VMs). Leave the 'Multiselect' option as 'No'. Save the dialog.
2. Now edit the dialog, and change the 'Multiselect' option to 'Yes'. Save the dialog

Actual results:

A pop-up error with the text:

URL https://cloudforms/api/service_dialogs/20 

Status 500 Internal Server Error 

Content-Type application/json; charset=utf-8 

Data {"error":{"kind":"internal_server_error","message":"undefined method `collect' for 1:Fixnum","klass":"NoMethodError"}} 

Expected results:
The dialog should be saved without error.

Additional info:
Attempting to edit the dialog now results in the same 500 error. The only thing that can be done to the dialog is to delete it.