BPMN2 Diagram Editor does not allow to input a variable for "called element" in CallActivity

Issue

In JBDS-8.1.0, BPMN2 Diagram Editor 1.1.2.Final does not allow to input a process variable for "called element" in CallActivity.

i.e.
1. create business process and then create call activity
2. create Reusable Process (Call Activity)
3. open property then go to Call Activity tab,
In the Called Element field, click pencil icon and then input process variable like #{processId}.

This can't be saved as OK button is grayed out if non alphabet character is included. (unable input numeric and special character, such as '#','{')

With either JBDS-7.1.1/jbdevstudio-integration-stack-updatesite-7.0.2.GA or Process Designer in Business-Central, user can input a variable and no validation error occurs. Also jBPM engine can operate a process which uses such variable for called element.